Introduction to SQL

SQL is a standard language for accessing and manipulating databases.

What is SQL?
• SQL stands for Structured Query Language
• SQL lets you access and manipulate databases
• SQL is an ANSI (American National Standards Institute) standard

What Can SQL do?

• SQL can execute queries against a database
• SQL can retrieve data from a database
• SQL can insert records in a database
• SQL can update records in a database
• SQL can delete records from a database
• SQL can create new databases
• SQL can create new tables in a database
• SQL can create stored procedures in a database
• SQL can create views in a database
• SQL can set permissions on tables, procedures, and views

SQL is a Standard - BUT....

Although SQL is an ANSI (American National Standards Institute) standard, there are many
different versions of the SQL language.

However, to be compliant with the ANSI standard, they all support at least the major commands
(such as SELECT, UPDATE, DELETE, INSERT, WHERE) in a similar manner.

Note: Most of the SQL database programs also have their own proprietary extensions in addition to
the SQL standard!

Using SQL in Your Web Site

To build a web site that shows some data from a database, you will need the following:

• An RDBMS database program (i.e. MS Access, SQL Server, MySQL)

• A server-side scripting language, like PHP or ASP
• SQL
• HTML / CSS
RDBMS
RDBMS stands for Relational Database Management System.

RDBMS is the basis for SQL, and for all modern database systems like MS SQL Server, IBM DB2,
Oracle, MySQL, and Microsoft Access.

The data in RDBMS is stored in database objects called tables.

A table is a collections of related data entries and it consists of columns and rows.

SQL Syntax
Database Tables
A database most often contains one or more tables. Each table is identified by a name (e.g.
"Customers" or "Orders"). Tables contain records (rows) with data.

Below is an example of a table called "Persons":

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

The table above contains three records (one for each person) and five columns (P_Id, LastName,
FirstName, Address, and City).

SQL Statements
Most of the actions you need to perform on a database are done with SQL statements.

The following SQL statement will select all the records in the "Persons" table:

SELECT * FROM Persons

In this tutorial we will teach you all about the different SQL statements.

Keep in Mind That...

• SQL is not case sensitive

Semicolon after SQL Statements?

Some database systems require a semicolon at the end of each SQL statement.

Semicolon is the standard way to separate each SQL statement in database systems that allow
more than one SQL statement to be executed in the same call to the server.

We are using MS Access and SQL Server 2000 and we do not have to put a semicolon after each
SQL statement, but some database programs force you to use it.
SQL DML and DDL
SQL can be divided into two parts: The Data Manipulation Language (DML) and the Data Definition
Language (DDL).

The query and update commands form the DML part of SQL:

• SELECT - extracts data from a database

• UPDATE - updates data in a database
• DELETE - deletes data from a database
• INSERT INTO - inserts new data into a database

The DDL part of SQL permits database tables to be created or deleted. It also define indexes (keys),
specify links between tables, and impose constraints between tables. The most important DDL
statements in SQL are:

• CREATE DATABASE - creates a new database

• ALTER DATABASE - modifies a database
• CREATE TABLE - creates a new table
• ALTER TABLE - modifies a table
• DROP TABLE - deletes a table
• CREATE INDEX - creates an index (search key)
• DROP INDEX - deletes an index
 SQL SELECT Statement

This chapter will explain the SELECT and the SELECT * statements.

The SQL SELECT Statement

The SELECT statement is used to select data from a database.

The result is stored in a result table, called the result-set.

SQL SELECT Syntax

SELECT column_name(s)
FROM table_name

and

SELECT * FROM table_name

Note: SQL is not case sensitive. SELECT is the same as select.

An SQL SELECT Example

The "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

Now we want to select the content of the columns named "LastName" and "FirstName" from the
table above.

We use the following SELECT statement:

SELECT LastName,FirstName FROM Persons

The result-set will look like this:

LastName FirstName
Hansen Ola
Svendson Tove
Pettersen Kari
SELECT * Example
Now we want to select all the columns from the "Persons" table.

We use the following SELECT statement:

SELECT * FROM Persons

Tip: The asterisk (*) is a quick way of selecting all columns!

The result-set will look like this:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

Navigation in a Result-set
Most database software systems allow navigation in the result-set with programming functions, like:
Move-To-First-Record, Get-Record-Content, Move-To-Next-Record, etc.

Programming functions like these are not a part of this tutorial. To learn about accessing data with
function calls, please visit our ADO tutorial or our PHP tutorial.

SQL SELECT DISTINCT Statement

This chapter will explain the SELECT DISTINCT statement.

The SQL SELECT DISTINCT Statement

In a table, some of the columns may contain duplicate values. This is not a problem, however,
sometimes you will want to list only the different (distinct) values in a table.

The DISTINCT keyword can be used to return only distinct (different) values.

SQL SELECT DISTINCT Syntax

SELECT DISTINCT column_name(s)
FROM table_name

SELECT DISTINCT Example

The "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

Now we want to select only the distinct values from the column named "City" from the table above.

We use the following SELECT statement:

SELECT DISTINCT City FROM Persons

The result-set will look like this:

City
Sandnes
Stavanger

SQL WHERE Clause

The WHERE clause is used to filter records.

The WHERE Clause
The WHERE clause is used to extract only those records that fulfill a specified criterion.

SQL WHERE Syntax

SELECT column_name(s)
FROM table_name
WHERE column_name operator value

WHERE Clause Example

The "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

Now we want to select only the persons living in the city "Sandnes" from the table above.

We use the following SELECT statement:

SELECT * FROM Persons

WHERE City='Sandnes'

The result-set will look like this:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes

Quotes Around Text Fields

SQL uses single quotes around text values (most database systems will also accept double quotes).

Although, numeric values should not be enclosed in quotes.

For text values:

This is correct:

SELECT * FROM Persons WHERE FirstName='Tove'

This is wrong:

SELECT * FROM Persons WHERE FirstName=Tove

For numeric values:

This is correct:

SELECT * FROM Persons WHERE Year=1965

This is wrong:

SELECT * FROM Persons WHERE Year='1965'

Operators Allowed in the WHERE Clause

With the WHERE clause, the following operators can be used:

Operator Description
= Equal
<> Not equal
> Greater than
< Less than
>= Greater than or equal
<= Less than or equal
BETWEEN Between an inclusive range
LIKE Search for a pattern
IN If you know the exact value you want to return for at least one of the columns

Note: In some versions of SQL the <> operator may be written as !=

SQL AND & OR Operators

The AND & OR operators are used to filter records based on more than one condition.

The AND & OR Operators

The AND operator displays a record if both the first condition and the second condition is true.

The OR operator displays a record if either the first condition or the second condition is true.
AND Operator Example
The "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

Now we want to select only the persons with the first name equal to "Tove" AND the last name
equal to "Svendson":

We use the following SELECT statement:

SELECT * FROM Persons

WHERE FirstName='Tove'
AND LastName='Svendson'

The result-set will look like this:

P_Id LastName FirstName Address City

2 Svendson Tove Borgvn 23 Sandnes

OR Operator Example
Now we want to select only the persons with the first name equal to "Tove" OR the first name equal
to "Ola":

We use the following SELECT statement:

SELECT * FROM Persons

WHERE FirstName='Tove'
OR FirstName='Ola'

The result-set will look like this:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes

Combining AND & OR

You can also combine AND and OR (use parenthesis to form complex expressions).

Now we want to select only the persons with the last name equal to "Svendson" AND the first name
equal to "Tove" OR to "Ola":
We use the following SELECT statement:

SELECT * FROM Persons WHERE

LastName='Svendson'
AND (FirstName='Tove' OR FirstName='Ola')

The result-set will look like this:

P_Id LastName FirstName Address City

2 Svendson Tove Borgvn 23 Sandnes

SQL ORDER BY Keyword

The ORDER BY keyword is used to sort the result-set.

The ORDER BY Keyword

The ORDER BY keyword is used to sort the result-set by a specified column.

The ORDER BY keyword sort the records in ascending order by default.

If you want to sort the records in a descending order, you can use the DESC keyword.
SQL ORDER BY Syntax
SELECT column_name(s)
FROM table_name
ORDER BY column_name(s) ASC|DESC

ORDER BY Example
The "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger
4 Nilsen Tom Vingvn 23 Stavanger

Now we want to select all the persons from the table above, however, we want to sort the persons
by their last name.

We use the following SELECT statement:

SELECT * FROM Persons

ORDER BY LastName

The result-set will look like this:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
4 Nilsen Tom Vingvn 23 Stavanger
3 Pettersen Kari Storgt 20 Stavanger
2 Svendson Tove Borgvn 23 Sandnes

ORDER BY DESC Example

Now we want to select all the persons from the table above, however, we want to sort the persons
descending by their last name.

We use the following SELECT statement:

SELECT * FROM Persons

ORDER BY LastName DESC

The result-set will look like this:

P_Id LastName FirstName Address City

2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger
4 Nilsen Tom Vingvn 23 Stavanger
1 Hansen Ola Timoteivn 10 Sandnes

SQL INSERT INTO Statement

The INSERT INTO statement is used to insert new records in a table.

The INSERT INTO Statement

The INSERT INTO statement is used to insert a new row in a table.

SQL INSERT INTO Syntax

It is possible to write the INSERT INTO statement in two forms.

The first form doesn't specify the column names where the data will be inserted, only their values:

INSERT INTO table_name

VALUES (value1, value2, value3,...)

The second form specifies both the column names and the values to be inserted:

INSERT INTO table_name (column1, column2, column3,...)

VALUES (value1, value2, value3,...)

SQL INSERT INTO Example

We have the following "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

Now we want to insert a new row in the "Persons" table.

We use the following SQL statement:

INSERT INTO Persons

VALUES (4,'Nilsen', 'Johan', 'Bakken 2', 'Stavanger')

The "Persons" table will now look like this:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger
4 Nilsen Johan Bakken 2 Stavanger

Insert Data Only in Specified Columns

It is also possible to only add data in specific columns.

The following SQL statement will add a new row, but only add data in the "P_Id", "LastName" and
the "FirstName" columns:

INSERT INTO Persons (P_Id, LastName, FirstName)

VALUES (5, 'Tjessem', 'Jakob')

The "Persons" table will now look like this:

P_Id LastName FirstName Address City
1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger
4 Nilsen Johan Bakken 2 Stavanger
5 Tjessem Jakob

SQL UPDATE Statement

The UPDATE statement is used to update records in a table.

The UPDATE Statement

The UPDATE statement is used to update existing records in a table.

SQL UPDATE Syntax

UPDATE table_name
SET column1=value, column2=value2,...
WHERE some_column=some_value
Note: Notice the WHERE clause in the UPDATE syntax. The WHERE clause specifies which record or
records that should be updated. If you omit the WHERE clause, all records will be updated!

SQL UPDATE Example

The "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger
4 Nilsen Johan Bakken 2 Stavanger
5 Tjessem Jakob

Now we want to update the person "Tjessem, Jakob" in the "Persons" table.

We use the following SQL statement:

UPDATE Persons
SET Address='Nissestien 67', City='Sandnes'
WHERE LastName='Tjessem' AND FirstName='Jakob'

The "Persons" table will now look like this:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger
4 Nilsen Johan Bakken 2 Stavanger
5 Tjessem Jakob Nissestien 67 Sandnes

SQL UPDATE Warning

Be careful when updating records. If we had omitted the WHERE clause in the example above, like
this:

UPDATE Persons
SET Address='Nissestien 67', City='Sandnes'

The "Persons" table would have looked like this:

P_Id LastName FirstName Address City

1 Hansen Ola Nissestien 67 Sandnes
2 Svendson Tove Nissestien 67 Sandnes
3 Pettersen Kari Nissestien 67 Sandnes
4 Nilsen Johan Nissestien 67 Sandnes
5 Tjessem Jakob Nissestien 67 Sandnes

SQL DELETE Statement

The DELETE statement is used to delete records in a table.

The DELETE Statement

The DELETE statement is used to delete rows in a table.

SQL DELETE Syntax

DELETE FROM table_name
WHERE some_column=some_value

Note: Notice the WHERE clause in the DELETE syntax. The WHERE clause specifies which record or
records that should be deleted. If you omit the WHERE clause, all records will be deleted!
SQL DELETE Example
The "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger
4 Nilsen Johan Bakken 2 Stavanger
5 Tjessem Jakob Nissestien 67 Sandnes

Now we want to delete the person "Tjessem, Jakob" in the "Persons" table.

We use the following SQL statement:

DELETE FROM Persons

WHERE LastName='Tjessem' AND FirstName='Jakob'

The "Persons" table will now look like this:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger
4 Nilsen Johan Bakken 2 Stavanger

Delete All Rows

It is possible to delete all rows in a table without deleting the table. This means that the table
structure, attributes, and indexes will be intact:

DELETE FROM table_name

or

DELETE * FROM table_name

Note: Be very careful when deleting records. You cannot undo this statement!
 SQL TOP Clause

The TOP Clause

The TOP clause is used to specify the number of records to return.

The TOP clause can be very useful on large tables with thousands of records. Returning a large
number of records can impact on performance.

Note: Not all database systems support the TOP clause.

SQL Server Syntax

SELECT TOP number|percent column_name(s)
FROM table_name
SQL SELECT TOP Equivalent in MySQL and Oracle
MySQL Syntax
SELECT column_name(s)
FROM table_name
LIMIT number

Example
SELECT *
FROM Persons
LIMIT 5

Oracle Syntax
SELECT column_name(s)
FROM table_name
WHERE ROWNUM <= number

Example
SELECT *
FROM Persons
WHERE ROWNUM <=5

SQL TOP Example

The "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger
4 Nilsen Tom Vingvn 23 Stavanger

Now we want to select only the two first records in the table above.

We use the following SELECT statement:

SELECT TOP 2 * FROM Persons

The result-set will look like this:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
SQL TOP PERCENT Example
The "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger
4 Nilsen Tom Vingvn 23 Stavanger

Now we want to select only 50% of the records in the table above.

We use the following SELECT statement:

SELECT TOP 50 PERCENT * FROM Persons

The result-set will look like this:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes

SQL LIKE Operator

The LIKE operator is used in a WHERE clause to search for a specified pattern in a
column.

The LIKE Operator

The LIKE operator is used to search for a specified pattern in a column.

SQL LIKE Syntax

SELECT column_name(s)
FROM table_name
WHERE column_name LIKE pattern
LIKE Operator Example
The "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

Now we want to select the persons living in a city that starts with "s" from the table above.

We use the following SELECT statement:

SELECT * FROM Persons

WHERE City LIKE 's%'

The "%" sign can be used to define wildcards (missing letters in the pattern) both before and after
the pattern.

The result-set will look like this:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

Next, we want to select the persons living in a city that ends with an "s" from the "Persons" table.

We use the following SELECT statement:

SELECT * FROM Persons

WHERE City LIKE '%s'

The result-set will look like this:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes

Next, we want to select the persons living in a city that contains the pattern "tav" from the
"Persons" table.

We use the following SELECT statement:

SELECT * FROM Persons

WHERE City LIKE '%tav%'

The result-set will look like this:

P_Id LastName FirstName Address City
3 Pettersen Kari Storgt 20 Stavanger

It is also possible to select the persons living in a city that NOT contains the pattern "tav" from the
"Persons" table, by using the NOT keyword.

We use the following SELECT statement:

SELECT * FROM Persons

WHERE City NOT LIKE '%tav%'

The result-set will look like this:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes

SQL Wildcards
SQL wildcards can be used when searching for data in a database.

SQL Wildcards
SQL wildcards can substitute for one or more characters when searching for data in a database.

SQL wildcards must be used with the SQL LIKE operator.

With SQL, the following wildcards can be used:

Wildcard Description
% A substitute for zero or more characters
_ A substitute for exactly one character
[charlist] Any single character in charlist
[^charlist] Any single character not in charlist

or
[!charlist]

SQL Wildcard Examples

We have the following "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

Using the % Wildcard

Now we want to select the persons living in a city that starts with "sa" from the "Persons" table.

We use the following SELECT statement:

SELECT * FROM Persons

WHERE City LIKE 'sa%'

The result-set will look like this:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes

Next, we want to select the persons living in a city that contains the pattern "nes" from the
"Persons" table.

We use the following SELECT statement:

SELECT * FROM Persons

WHERE City LIKE '%nes%'

The result-set will look like this:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes

Using the _ Wildcard

Now we want to select the persons with a first name that starts with any character, followed by "la"
from the "Persons" table.
We use the following SELECT statement:

SELECT * FROM Persons

WHERE FirstName LIKE '_la'

The result-set will look like this:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes

Next, we want to select the persons with a last name that starts with "S", followed by any character,
followed by "end", followed by any character, followed by "on" from the "Persons" table.

We use the following SELECT statement:

SELECT * FROM Persons

WHERE LastName LIKE 'S_end_on'

The result-set will look like this:

P_Id LastName FirstName Address City

2 Svendson Tove Borgvn 23 Sandnes

Using the [charlist] Wildcard

Now we want to select the persons with a last name that starts with "b" or "s" or "p" from the
"Persons" table.

We use the following SELECT statement:

SELECT * FROM Persons

WHERE LastName LIKE '[bsp]%'

The result-set will look like this:

P_Id LastName FirstName Address City

2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

Next, we want to select the persons with a last name that do not start with "b" or "s" or "p" from
the "Persons" table.

We use the following SELECT statement:

SELECT * FROM Persons

WHERE LastName LIKE '[!bsp]%'

The result-set will look like this:

P_Id LastName FirstName Address City
1 Hansen Ola Timoteivn 10 Sandnes

SQL IN Operator

The IN Operator
The IN operator allows you to specify multiple values in a WHERE clause.

SQL IN Syntax
SELECT column_name(s)
FROM table_name
WHERE column_name IN (value1,value2,...)

IN Operator Example
The "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

Now we want to select the persons with a last name equal to "Hansen" or "Pettersen" from the table
above.

We use the following SELECT statement:

SELECT * FROM Persons
WHERE LastName IN ('Hansen','Pettersen')

The result-set will look like this:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

SQL BETWEEN Operator

The BETWEEN operator is used in a WHERE clause to select a range of data between
two values.

The BETWEEN Operator

The BETWEEN operator selects a range of data between two values. The values can be numbers,
text, or dates.

SQL BETWEEN Syntax

SELECT column_name(s)
FROM table_name
WHERE column_name
BETWEEN value1 AND value2

BETWEEN Operator Example

The "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

Now we want to select the persons with a last name alphabetically between "Hansen" and
"Pettersen" from the table above.

We use the following SELECT statement:

SELECT * FROM Persons
WHERE LastName
BETWEEN 'Hansen' AND 'Pettersen'

The result-set will look like this:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes

Note: The BETWEEN operator is treated differently in different databases.

In some databases, persons with the LastName of "Hansen" or "Pettersen" will not be listed,
because the BETWEEN operator only selects fields that are between and excluding the test values).

In other databases, persons with the LastName of "Hansen" or "Pettersen" will be listed, because
the BETWEEN operator selects fields that are between and including the test values).

And in other databases, persons with the LastName of "Hansen" will be listed, but "Pettersen" will
not be listed (like the example above), because the BETWEEN operator selects fields between the
test values, including the first test value and excluding the last test value.

Therefore: Check how your database treats the BETWEEN operator.

Example 2
To display the persons outside the range in the previous example, use NOT BETWEEN:

SELECT * FROM Persons

WHERE LastName
NOT BETWEEN 'Hansen' AND 'Pettersen'

The result-set will look like this:

P_Id LastName FirstName Address City

2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger
 SQL Alias
With SQL, an alias name can be given to a table or to a column.

SQL Alias
You can give a table or a column another name by using an alias. This can be a good thing to do if
you have very long or complex table names or column names.

An alias name could be anything, but usually it is short.

SQL Alias Syntax for Tables

SELECT column_name(s)
FROM table_name
AS alias_name

SQL Alias Syntax for Columns

SELECT column_name AS alias_name
FROM table_name

Alias Example
Assume we have a table called "Persons" and another table called "Product_Orders". We will give
the table aliases of "p" an "po" respectively.

Now we want to list all the orders that "Ola Hansen" is responsible for.

We use the following SELECT statement:

SELECT po.OrderID, p.LastName, p.FirstName

FROM Persons AS p,
Product_Orders AS po
WHERE p.LastName='Hansen'
WHERE p.FirstName='Ola'

The same SELECT statement without aliases:

SELECT Product_Orders.OrderID, Persons.LastName, Persons.FirstName

FROM Persons,
Product_Orders
WHERE Persons.LastName='Hansen'
WHERE Persons.FirstName='Ola'

As you'll see from the two SELECT statements above; aliases can make queries easier to both write
and to read.

SQL Joins
SQL joins are used to query data from two or more tables, based on a relationship
between certain columns in these tables.

SQL JOIN
The JOIN keyword is used in an SQL statement to query data from two or more tables, based on a
relationship between certain columns in these tables.

Tables in a database are often related to each other with keys.

A primary key is a column (or a combination of columns) with a unique value for each row. Each
primary key value must be unique within the table. The purpose is to bind data together, across
tables, without repeating all of the data in every table.

Look at the "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

Note that the "P_Id" column is the primary key in the "Persons" table. This means that no two rows
can have the same P_Id. The P_Id distinguishes two persons even if they have the same name.

Next, we have the "Orders" table:

O_Id OrderNo P_Id

1 77895 3
2 44678 3
3 22456 1
4 24562 1
5 34764 15

Note that the "O_Id" column is the primary key in the "Orders" table and that the "P_Id" column
refers to the persons in the "Persons" table without using their names.

Notice that the relationship between the two tables above is the "P_Id" column.

Different SQL JOINs

Before we continue with examples, we will list the types of JOIN you can use, and the differences
between them.

• JOIN: Return rows when there is at least one match in both tables
• LEFT JOIN: Return all rows from the left table, even if there are no matches in the right
table
• RIGHT JOIN: Return all rows from the right table, even if there are no matches in the left
table
• FULL JOIN: Return rows when there is a match in one of the tables
 SQL INNER JOIN Keyword
SQL INNER JOIN Keyword
The INNER JOIN keyword return rows when there is at least one match in both tables.

SQL INNER JOIN Syntax

SELECT column_name(s)
FROM table_name1
INNER JOIN table_name2
ON table_name1.column_name=table_name2.column_name

PS: INNER JOIN is the same as JOIN.

SQL INNER JOIN Example

The "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

The "Orders" table:

O_Id OrderNo P_Id

1 77895 3
2 44678 3
3 22456 1
4 24562 1
5 34764 15

Now we want to list all the persons with any orders.

We use the following SELECT statement:

SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo

FROM Persons
INNER JOIN Orders
ON Persons.P_Id=Orders.P_Id
ORDER BY Persons.LastName

The result-set will look like this:

LastName FirstName OrderNo

Hansen Ola 22456
Hansen Ola 24562
Pettersen Kari 77895
Pettersen Kari 44678

The INNER JOIN keyword return rows when there is at least one match in both tables. If there are
rows in "Persons" that do not have matches in "Orders", those rows will NOT be listed.
 SQL LEFT JOIN Keyword
SQL LEFT JOIN Keyword
The LEFT JOIN keyword returns all rows from the left table (table_name1), even if there are no
matches in the right table (table_name2).

SQL LEFT JOIN Syntax

SELECT column_name(s)
FROM table_name1
LEFT JOIN table_name2
ON table_name1.column_name=table_name2.column_name

PS: In some databases LEFT JOIN is called LEFT OUTER JOIN.

SQL LEFT JOIN Example

The "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

The "Orders" table:

O_Id OrderNo P_Id

1 77895 3
2 44678 3
3 22456 1
4 24562 1
5 34764 15

Now we want to list all the persons and their orders - if any, from the tables above.
We use the following SELECT statement:

SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo

FROM Persons
LEFT JOIN Orders
ON Persons.P_Id=Orders.P_Id
ORDER BY Persons.LastName

The result-set will look like this:

LastName FirstName OrderNo

Hansen Ola 22456
Hansen Ola 24562
Pettersen Kari 77895
Pettersen Kari 44678
Svendson Tove

The LEFT JOIN keyword returns all the rows from the left table (Persons), even if there are no
matches in the right table (Orders).
 SQL RIGHT JOIN Keyword

SQL RIGHT JOIN Keyword

The RIGHT JOIN keyword Return all rows from the right table (table_name2), even if there are no
matches in the left table (table_name1).

SQL RIGHT JOIN Syntax

SELECT column_name(s)
FROM table_name1
RIGHT JOIN table_name2
ON table_name1.column_name=table_name2.column_name

PS: In some databases RIGHT JOIN is called RIGHT OUTER JOIN.

SQL RIGHT JOIN Example

The "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

The "Orders" table:

O_Id OrderNo P_Id

1 77895 3
2 44678 3
3 22456 1
4 24562 1
5 34764 15

Now we want to list all the orders with containing persons - if any, from the tables above.

We use the following SELECT statement:

SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo
FROM Persons
RIGHT JOIN Orders
ON Persons.P_Id=Orders.P_Id
ORDER BY Persons.LastName

The result-set will look like this:

LastName FirstName OrderNo

Hansen Ola 22456
Hansen Ola 24562
Pettersen Kari 77895
Pettersen Kari 44678
34764

The RIGHT JOIN keyword returns all the rows from the right table (Orders), even if there are no
matches in the left table (Persons).
 SQL FULL JOIN Keyword
SQL FULL JOIN Keyword
The FULL JOIN keyword return rows when there is a match in one of the tables.

SQL FULL JOIN Syntax

SELECT column_name(s)
FROM table_name1
FULL JOIN table_name2
ON table_name1.column_name=table_name2.column_name

SQL FULL JOIN Example

The "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

The "Orders" table:

O_Id OrderNo P_Id

1 77895 3
2 44678 3
3 22456 1
4 24562 1
5 34764 15

Now we want to list all the persons and their orders, and all the orders with their persons.

We use the following SELECT statement:

SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo

FROM Persons
FULL JOIN Orders
ON Persons.P_Id=Orders.P_Id
ORDER BY Persons.LastName

The result-set will look like this:

LastName FirstName OrderNo
Hansen Ola 22456
Hansen Ola 24562
Pettersen Kari 77895
Pettersen Kari 44678
Svendson Tove
34764

The FULL JOIN keyword returns all the rows from the left table (Persons), and all the rows from the
right table (Orders). If there are rows in "Persons" that do not have matches in "Orders", or if there
are rows in "Orders" that do not have matches in "Persons", those rows will be listed as well.
 SQL UNION Operator
The SQL UNION operator combines two or more SELECT statements.

The SQL UNION Operator

The UNION operator is used to combine the result-set of two or more SELECT statements.

Notice that each SELECT statement within the UNION must have the same number of columns. The
columns must also have similar data types. Also, the columns in each SELECT statement must be in
the same order.

SQL UNION Syntax

SELECT column_name(s) FROM table_name1
UNION
SELECT column_name(s) FROM table_name2

Note: The UNION operator selects only distinct values by default. To allow duplicate values, use
UNION ALL.

SQL UNION ALL Syntax

SELECT column_name(s) FROM table_name1
UNION ALL
SELECT column_name(s) FROM table_name2

PS: The column names in the result-set of a UNION are always equal to the column names in the
first SELECT statement in the UNION.

SQL UNION Example

Look at the following tables:

"Employees_Norway":

E_ID E_Name
01 Hansen, Ola
02 Svendson, Tove
03 Svendson, Stephen
04 Pettersen, Kari

"Employees_USA":

E_ID E_Name
01 Turner, Sally
02 Kent, Clark
03 Svendson, Stephen
04 Scott, Stephen

Now we want to list all the different employees in Norway and USA.

We use the following SELECT statement:

SELECT E_Name FROM Employees_Norway

UNION
SELECT E_Name FROM Employees_USA

The result-set will look like this:

E_Name
Hansen, Ola
Svendson, Tove
Svendson, Stephen
Pettersen, Kari
Turner, Sally
Kent, Clark
Scott, Stephen

Note: This command cannot be used to list all employees in Norway and USA. In the example
above we have two employees with equal names, and only one of them will be listed. The UNION
command selects only distinct values.

SQL UNION ALL Example

Now we want to list all employees in Norway and USA:

SELECT E_Name FROM Employees_Norway

UNION ALL
SELECT E_Name FROM Employees_USA

Result

E_Name
Hansen, Ola
Svendson, Tove
Svendson, Stephen
Pettersen, Kari
Turner, Sally
Kent, Clark
Svendson, Stephen
Scott, Stephen
 SQL SELECT INTO Statement

The SQL SELECT INTO statement can be used to create backup copies of tables.

The SQL SELECT INTO Statement

The SELECT INTO statement selects data from one table and inserts it into a different table.

The SELECT INTO statement is most often used to create backup copies of tables.

SQL SELECT INTO Syntax

We can select all columns into the new table:

SELECT *
INTO new_table_name [IN externaldatabase]
FROM old_tablename

Or we can select only the columns we want into the new table:

SELECT column_name(s)
INTO new_table_name [IN externaldatabase]
FROM old_tablename

SQL SELECT INTO Example

Make a Backup Copy - Now we want to make an exact copy of the data in our "Persons" table.

We use the following SQL statement:

SELECT *
INTO Persons_Backup
FROM Persons

We can also use the IN clause to copy the table into another database:

SELECT *
INTO Persons_Backup IN 'Backup.mdb'
FROM Persons

We can also copy only a few fields into the new table:

SELECT LastName,FirstName
INTO Persons_Backup
FROM Persons
SQL SELECT INTO - With a WHERE Clause
We can also add a WHERE clause.

The following SQL statement creates a "Persons_Backup" table with only the persons who lives in
the city "Sandnes":

SELECT LastName,Firstname
INTO Persons_Backup
FROM Persons
WHERE City='Sandnes'

SQL SELECT INTO - Joined Tables

Selecting data from more than one table is also possible.

The following example creates a "Persons_Order_Backup" table contains data from the two tables
"Persons" and "Orders":

SELECT Persons.LastName,Orders.OrderNo
INTO Persons_Order_Backup
FROM Persons
INNER JOIN Orders
ON Persons.P_Id=Orders.P_Id

SQL CREATE DATABASE Statement

The CREATE DATABASE Statement
The CREATE DATABASE statement is used to create a database.

SQL CREATE DATABASE Syntax

CREATE DATABASE database_name

CREATE DATABASE Example

Now we want to create a database called "my_db".

We use the following CREATE DATABASE statement:

CREATE DATABASE my_db

Database tables can be added with the CREATE TABLE statement.

SQL CREATE TABLE Statement

The CREATE TABLE Statement
The CREATE TABLE statement is used to create a table in a database.

SQL CREATE TABLE Syntax

CREATE TABLE table_name
(
column_name1 data_type,
column_name2 data_type,
column_name3 data_type,
....
)

The data type specifies what type of data the column can hold. For a complete reference of all the
data types available in MS Access, MySQL, and SQL Server, go to our complete Data Types
reference.

CREATE TABLE Example

Now we want to create a table called "Persons" that contains five columns: P_Id, LastName,
FirstName, Address, and City.

We use the following CREATE TABLE statement:

CREATE TABLE Persons

(
P_Id int,
LastName varchar(255),
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)

The P_Id column is of type int and will hold a number. The LastName, FirstName, Address, and City
columns are of type varchar with a maximum length of 255 characters.

The empty "Persons" table will now look like this:

P_Id LastName FirstName Address City

The empty table can be filled with data with the INSERT INTO statement.

SQL Constraints
SQL Constraints
Constraints are used to limit the type of data that can go into a table.

Constraints can be specified when a table is created (with the CREATE TABLE statement) or after the
table is created (with the ALTER TABLE statement).

We will focus on the following constraints:

• NOT NULL
• UNIQUE
• PRIMARY KEY
• FOREIGN KEY
• CHECK
• DEFAULT

The next chapters will describe each constraint in details.

SQL NOT NULL Constraint

SQL NOT NULL Constraint

The NOT NULL constraint enforces a column to NOT accept NULL values.

The NOT NULL constraint enforces a field to always contain a value. This means that you cannot
insert a new record, or update a record without adding a value to this field.

The following SQL enforces the "P_Id" column and the "LastName" column to not accept NULL
values:

CREATE TABLE Persons

(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)

SQL UNIQUE Constraint

SQL UNIQUE Constraint
The UNIQUE constraint uniquely identifies each record in a database table.

The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or
set of columns.

A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it.

Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint
per table.

SQL UNIQUE Constraint on CREATE TABLE

The following SQL creates a UNIQUE constraint on the "P_Id" column when the "Persons" table is
created:

MySQL:

CREATE TABLE Persons

(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
UNIQUE (P_Id)
)

SQL Server / Oracle / MS Access:

CREATE TABLE Persons

(
P_Id int NOT NULL UNIQUE,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)

To allow naming of a UNIQUE constraint, and for defining a UNIQUE constraint on multiple columns,
use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE Persons

(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)
)

SQL UNIQUE Constraint on ALTER TABLE

To create a UNIQUE constraint on the "P_Id" column when the table is already created, use the
following SQL:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons

ADD UNIQUE (P_Id)

To allow naming of a UNIQUE constraint, and for defining a UNIQUE constraint on multiple columns,
use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons

ADD CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)

To DROP a UNIQUE Constraint

To drop a UNIQUE constraint, use the following SQL:

MySQL:

ALTER TABLE Persons

DROP INDEX uc_PersonID

SQL Server / Oracle / MS Access:

ALTER TABLE Persons

DROP CONSTRAINT uc_PersonID

SQL PRIMARY KEY Constraint

SQL PRIMARY KEY Constraint
The PRIMARY KEY constraint uniquely identifies each record in a database table.

Primary keys must contain unique values.

A primary key column cannot contain NULL values.

Each table should have a primary key, and each table can have only one primary key.

SQL PRIMARY KEY Constraint on CREATE TABLE

The following SQL creates a PRIMARY KEY on the "P_Id" column when the "Persons" table is
created:

MySQL:

CREATE TABLE Persons

(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
PRIMARY KEY (P_Id)
)

SQL Server / Oracle / MS Access:

CREATE TABLE Persons

(
P_Id int NOT NULL PRIMARY KEY,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)

To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on
multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE Persons

(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)
)
SQL PRIMARY KEY Constraint on ALTER TABLE
To create a PRIMARY KEY constraint on the "P_Id" column when the table is already created, use the
following SQL:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons

ADD PRIMARY KEY (P_Id)

To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on
multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons

ADD CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)

Note: If you use the ALTER TABLE statement to add a primary key, the primary key column(s) must
already have been declared to not contain NULL values (when the table was first created).

To DROP a PRIMARY KEY Constraint

To drop a PRIMARY KEY constraint, use the following SQL:

MySQL:

ALTER TABLE Persons

DROP PRIMARY KEY

SQL Server / Oracle / MS Access:

ALTER TABLE Persons

DROP CONSTRAINT pk_PersonID

SQL FOREIGN KEY Constraint

SQL FOREIGN KEY Constraint

A FOREIGN KEY in one table points to a PRIMARY KEY in another table.
Let's illustrate the foreign key with an example. Look at the following two tables:

The "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

The "Orders" table:

O_Id OrderNo P_Id

1 77895 3
2 44678 3
3 22456 2
4 24562 1

Note that the "P_Id" column in the "Orders" table points to the "P_Id" column in the "Persons" table.

The "P_Id" column in the "Persons" table is the PRIMARY KEY in the "Persons" table.

The "P_Id" column in the "Orders" table is a FOREIGN KEY in the "Orders" table.

The FOREIGN KEY constraint is used to prevent actions that would destroy link between tables.

The FOREIGN KEY constraint also prevents that invalid data is inserted into the foreign key column,
because it has to be one of the values contained in the table it points to.

SQL FOREIGN KEY Constraint on CREATE TABLE

The following SQL creates a FOREIGN KEY on the "P_Id" column when the "Orders" table is created:

MySQL:

CREATE TABLE Orders

(
O_Id int NOT NULL,
OrderNo int NOT NULL,
P_Id int,
PRIMARY KEY (O_Id),
FOREIGN KEY (P_Id) REFERENCES Persons(P_Id)
)

SQL Server / Oracle / MS Access:

CREATE TABLE Orders

(
O_Id int NOT NULL PRIMARY KEY,
OrderNo int NOT NULL,
P_Id int FOREIGN KEY REFERENCES Persons(P_Id)
)

To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on
multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE Orders

(
O_Id int NOT NULL,
OrderNo int NOT NULL,
P_Id int,
PRIMARY KEY (O_Id),
CONSTRAINT fk_PerOrders FOREIGN KEY (P_Id)
REFERENCES Persons(P_Id)
)

SQL FOREIGN KEY Constraint on ALTER TABLE

To create a FOREIGN KEY constraint on the "P_Id" column when the "Orders" table is already
created, use the following SQL:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Orders

ADD FOREIGN KEY (P_Id)
REFERENCES Persons(P_Id)

To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on
multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Orders

ADD CONSTRAINT fk_PerOrders
FOREIGN KEY (P_Id)
REFERENCES Persons(P_Id)

To DROP a FOREIGN KEY Constraint

To drop a FOREIGN KEY constraint, use the following SQL:

MySQL:

ALTER TABLE Orders

DROP FOREIGN KEY fk_PerOrders

SQL Server / Oracle / MS Access:

ALTER TABLE Orders
DROP CONSTRAINT fk_PerOrders

SQL CHECK Constraint

SQL CHECK Constraint

The CHECK constraint is used to limit the value range that can be placed in a column.
If you define a CHECK constraint on a single column it allows only certain values for this column.

If you define a CHECK constraint on a table it can limit the values in certain columns based on
values in other columns in the row.

SQL CHECK Constraint on CREATE TABLE

The following SQL creates a CHECK constraint on the "P_Id" column when the "Persons" table is
created. The CHECK constraint specifies that the column "P_Id" must only include integers greater
than 0.

My SQL:

CREATE TABLE Persons

(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CHECK (P_Id>0)
)

SQL Server / Oracle / MS Access:

CREATE TABLE Persons

(
P_Id int NOT NULL CHECK (P_Id>0),
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)

To allow naming of a CHECK constraint, and for defining a CHECK constraint on multiple columns,
use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE Persons

(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT chk_Person CHECK (P_Id>0 AND City='Sandnes')
)
SQL CHECK Constraint on ALTER TABLE
To create a CHECK constraint on the "P_Id" column when the table is already created, use the
following SQL:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons

ADD CHECK (P_Id>0)

To allow naming of a CHECK constraint, and for defining a CHECK constraint on multiple columns,
use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons

ADD CONSTRAINT chk_Person CHECK (P_Id>0 AND City='Sandnes')

To DROP a CHECK Constraint

To drop a CHECK constraint, use the following SQL:

SQL Server / Oracle / MS Access:

ALTER TABLE Persons

DROP CONSTRAINT chk_Person

SQL DEFAULT Constraint

SQL DEFAULT Constraint
The DEFAULT constraint is used to insert a default value into a column.

The default value will be added to all new records, if no other value is specified.
SQL DEFAULT Constraint on CREATE TABLE
The following SQL creates a DEFAULT constraint on the "City" column when the "Persons" table is
created:

My SQL / SQL Server / Oracle / MS Access:

CREATE TABLE Persons

(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255) DEFAULT 'Sandnes'
)

The DEFAULT constraint can also be used to insert system values, by using functions like
GETDATE():

CREATE TABLE Orders

(
O_Id int NOT NULL,
OrderNo int NOT NULL,
P_Id int,
OrderDate date DEFAULT GETDATE()
)

SQL DEFAULT Constraint on ALTER TABLE

To create a DEFAULT constraint on the "City" column when the table is already created, use the
following SQL:

MySQL:

ALTER TABLE Persons

ALTER City SET DEFAULT 'SANDNES'

SQL Server / Oracle / MS Access:

ALTER TABLE Persons

ALTER COLUMN City SET DEFAULT 'SANDNES'

To DROP a DEFAULT Constraint

To drop a DEFAULT constraint, use the following SQL:

MySQL:

ALTER TABLE Persons

ALTER City DROP DEFAULT

SQL Server / Oracle / MS Access:

ALTER TABLE Persons

ALTER COLUMN City DROP DEFAULT

SQL CREATE INDEX Statement

The CREATE INDEX statement is used to create indexes in tables.

Indexes allow the database application to find data fast; without reading the whole
table.

Indexes
An index can be created in a table to find data more quickly and efficiently.
The users cannot see the indexes, they are just used to speed up searches/queries.

Note: Updating a table with indexes takes more time than updating a table without (because the
indexes also need an update). So you should only create indexes on columns (and tables) that will
be frequently searched against.

SQL CREATE INDEX Syntax

Creates an index on a table. Duplicate values are allowed:

CREATE INDEX index_name

ON table_name (column_name)

SQL CREATE UNIQUE INDEX Syntax

Creates a unique index on a table. Duplicate values are not allowed:

CREATE UNIQUE INDEX index_name

ON table_name (column_name)

Note: The syntax for creating indexes varies amongst different databases. Therefore: Check the
syntax for creating indexes in your database.

CREATE INDEX Example

The SQL statement below creates an index named "PIndex" on the "LastName" column in the
"Persons" table:

CREATE INDEX PIndex

ON Persons (LastName)

If you want to create an index on a combination of columns, you can list the column names within
the parentheses, separated by commas:

CREATE INDEX PIndex

ON Persons (LastName, FirstName)
 SQL DROP INDEX, DROP TABLE, and DROP
DATABASE

Indexes, tables, and databases can easily be deleted/removed with the DROP
statement.

The DROP INDEX Statement

The DROP INDEX statement is used to delete an index in a table.
DROP INDEX Syntax for MS Access:
DROP INDEX index_name ON table_name

DROP INDEX Syntax for MS SQL Server:

DROP INDEX table_name.index_name

DROP INDEX Syntax for DB2/Oracle:

DROP INDEX index_name

DROP INDEX Syntax for MySQL:

ALTER TABLE table_name DROP INDEX index_name

The DROP TABLE Statement

The DROP TABLE statement is used to delete a table.

DROP TABLE table_name

The DROP DATABASE Statement

The DROP DATABASE statement is used to delete a database.

DROP DATABASE database_name

The TRUNCATE TABLE Statement

What if we only want to delete the data inside the table, and not the table itself?

Then, use the TRUNCATE TABLE statement:

TRUNCATE TABLE table_name

 SQL ALTER TABLE Statement

The ALTER TABLE Statement

The ALTER TABLE statement is used to add, delete, or modify columns in an existing table.

SQL ALTER TABLE Syntax

To add a column in a table, use the following syntax:

ALTER TABLE table_name

ADD column_name datatype

To delete a column in a table, use the following syntax (notice that some database systems don't
allow deleting a column):
ALTER TABLE table_name
DROP COLUMN column_name

To change the data type of a column in a table, use the following syntax:

ALTER TABLE table_name

ALTER COLUMN column_name datatype

SQL ALTER TABLE Example

Look at the "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

Now we want to add a column named "DateOfBirth" in the "Persons" table.

We use the following SQL statement:

ALTER TABLE Persons

ADD DateOfBirth date

Notice that the new column, "DateOfBirth", is of type date and is going to hold a date. The data
type specifies what type of data the column can hold. For a complete reference of all the data types
available in MS Access, MySQL, and SQL Server, go to our complete Data Types reference.

The "Persons" table will now like this:

P_Id LastName FirstName Address City DateOfBirth

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

Change Data Type Example

Now we want to change the data type of the column named "DateOfBirth" in the "Persons" table.

We use the following SQL statement:

ALTER TABLE Persons

ALTER COLUMN DateOfBirth year

Notice that the "DateOfBirth" column is now of type year and is going to hold a year in a two-digit
or four-digit format.
DROP COLUMN Example
Next, we want to delete the column named "DateOfBirth" in the "Persons" table.

We use the following SQL statement:

ALTER TABLE Persons

DROP COLUMN DateOfBirth

The "Persons" table will now like this:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

SQL AUTO INCREMENT Field

Auto-increment allows a unique number to be generated when a new record is

inserted into a table.

AUTO INCREMENT a Field

Very often we would like the value of the primary key field to be created automatically every time a
new record is inserted.

We would like to create an auto-increment field in a table.

Syntax for MySQL

The following SQL statement defines the "P_Id" column to be an auto-increment primary key field in
the "Persons" table:
CREATE TABLE Persons
(
P_Id int NOT NULL AUTO_INCREMENT,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
PRIMARY KEY (P_Id)
)

MySQL uses the AUTO_INCREMENT keyword to perform an auto-increment feature.

By default, the starting value for AUTO_INCREMENT is 1, and it will increment by 1 for each new
record.

To let the AUTO_INCREMENT sequence start with another value, use the following SQL statement:

ALTER TABLE Persons AUTO_INCREMENT=100

To insert a new record into the "Persons" table, we will not have to specify a value for the "P_Id"
column (a unique value will be added automatically):

INSERT INTO Persons (FirstName,LastName)

VALUES ('Lars','Monsen')

The SQL statement above would insert a new record into the "Persons" table. The "P_Id" column
would be assigned a unique value. The "FirstName" column would be set to "Lars" and the
"LastName" column would be set to "Monsen".

Syntax for SQL Server

The following SQL statement defines the "P_Id" column to be an auto-increment primary key field in
the "Persons" table:

CREATE TABLE Persons

(
P_Id int PRIMARY KEY IDENTITY,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)

The MS SQL Server uses the IDENTITY keyword to perform an auto-increment feature.

By default, the starting value for IDENTITY is 1, and it will increment by 1 for each new record.

To specify that the "P_Id" column should start at value 10 and increment by 5, change the identity
to IDENTITY(10,5).

To insert a new record into the "Persons" table, we will not have to specify a value for the "P_Id"
column (a unique value will be added automatically):
INSERT INTO Persons (FirstName,LastName)
VALUES ('Lars','Monsen')

The SQL statement above would insert a new record into the "Persons" table. The "P_Id" column
would be assigned a unique value. The "FirstName" column would be set to "Lars" and the
"LastName" column would be set to "Monsen".

Syntax for Access

The following SQL statement defines the "P_Id" column to be an auto-increment primary key field in
the "Persons" table:

CREATE TABLE Persons

(
P_Id PRIMARY KEY AUTOINCREMENT,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)

The MS Access uses the AUTOINCREMENT keyword to perform an auto-increment feature.

By default, the starting value for AUTOINCREMENT is 1, and it will increment by 1 for each new
record.

To specify that the "P_Id" column should start at value 10 and increment by 5, change the
autoincrement to AUTOINCREMENT(10,5).

To insert a new record into the "Persons" table, we will not have to specify a value for the "P_Id"
column (a unique value will be added automatically):

INSERT INTO Persons (FirstName,LastName)

VALUES ('Lars','Monsen')

The SQL statement above would insert a new record into the "Persons" table. The "P_Id" column
would be assigned a unique value. The "FirstName" column would be set to "Lars" and the
"LastName" column would be set to "Monsen".

Syntax for Oracle

In Oracle the code is a little bit more tricky.

You will have to create an auto-increment field with the sequence object (this object generates a
number sequence).

Use the following CREATE SEQUENCE syntax:

CREATE SEQUENCE seq_person

MINVALUE 1
START WITH 1
INCREMENT BY 1
CACHE 10

The code above creates a sequence object called seq_person, that starts with 1 and will increment
by 1. It will also cache up to 10 values for performance. The cache option specifies how many
sequence values will be stored in memory for faster access.

To insert a new record into the "Persons" table, we will have to use the nextval function (this
function retrieves the next value from seq_person sequence):

INSERT INTO Persons (P_Id,FirstName,LastName)

VALUES (seq_person.nextval,'Lars','Monsen')

The SQL statement above would insert a new record into the "Persons" table. The "P_Id" column
would be assigned the next number from the seq_person sequence. The "FirstName" column would
be set to "Lars" and the "LastName" column would be set to "Monsen".

SQL Views

A view is a virtual table.

This chapter shows how to create, update, and delete a view.

SQL CREATE VIEW Statement

In SQL, a view is a virtual table based on the result-set of an SQL statement.

A view contains rows and columns, just like a real table. The fields in a view are fields from one
or more real tables in the database.

You can add SQL functions, WHERE, and JOIN statements to a view and present the data as if
the data were coming from one single table.

SQL CREATE VIEW Syntax

CREATE VIEW view_name AS

SELECT column_name(s)
FROM table_name
WHERE condition
Note: A view always shows up-to-date data! The database engine recreates the data, using the
view's SQL statement, every time a user queries a view.
SQL CREATE VIEW Examples
If you have the Northwind database you can see that it has several views installed by default.

The view "Current Product List" lists all active products (products that are not discontinued)
from the "Products" table. The view is created with the following SQL:

CREATE VIEW [Current Product List] AS

SELECT ProductID,ProductName
FROM Products
WHERE Discontinued=No
We can query the view above as follows:

SELECT * FROM [Current Product List]

Another view in the Northwind sample database selects every product in the "Products" table
with a unit price higher than the average unit price:

CREATE VIEW [Products Above Average Price] AS

SELECT ProductName,UnitPrice
FROM Products
WHERE UnitPrice>(SELECT AVG(UnitPrice) FROM Products)
We can query the view above as follows:

SELECT * FROM [Products Above Average Price]

Another view in the Northwind database calculates the total sale for each category in 1997.
Note that this view selects its data from another view called "Product Sales for 1997":

CREATE VIEW [Category Sales For 1997] AS

SELECT DISTINCT CategoryName,Sum(ProductSales) AS CategorySales
FROM [Product Sales for 1997]
GROUP BY CategoryName
We can query the view above as follows:

SELECT * FROM [Category Sales For 1997]

We can also add a condition to the query. Now we want to see the total sale only for the
category "Beverages":

SELECT * FROM [Category Sales For 1997]

WHERE CategoryName='Beverages'

SQL Updating a View

You can update a view by using the following syntax:

SQL CREATE OR REPLACE VIEW Syntax

CREATE OR REPLACE VIEW view_name AS
SELECT column_name(s)
FROM table_name
WHERE condition
Now we want to add the "Category" column to the "Current Product List" view. We will update
the view with the following SQL:

CREATE VIEW [Current Product List] AS

SELECT ProductID,ProductName,Category
FROM Products
WHERE Discontinued=No

SQL Dropping a View

You can delete a view with the DROP VIEW command.

SQL DROP VIEW Syntax

DROP VIEW view_name
 SQL Date Functions

SQL Dates

The most difficult part when working with dates is to be sure that the format of the date you are
trying to insert, matches the format of the date column in the database.

As long as your data contains only the date portion, your queries will work as expected. However, if
a time portion is involved, it gets complicated.

Before talking about the complications of querying for dates, we will look at the most important
built-in functions for working with dates.

MySQL Date Functions

The following table lists the most important built-in date functions in MySQL:

Function Description
NOW() Returns the current date and time
CURDATE() Returns the current date
CURTIME() Returns the current time
DATE() Extracts the date part of a date or date/time expression
EXTRACT() Returns a single part of a date/time
DATE_ADD() Adds a specified time interval to a date
DATE_SUB() Subtracts a specified time interval from a date
DATEDIFF() Returns the number of days between two dates
DATE_FORMAT() Displays date/time data in different formats

SQL Server Date Functions

The following table lists the most important built-in date functions in SQL Server:
Function Description
GETDATE() Returns the current date and time
DATEPART() Returns a single part of a date/time
DATEADD() Adds or subtracts a specified time interval from a date
DATEDIFF() Returns the time between two dates
CONVERT() Displays date/time data in different formats

SQL Date Data Types

MySQL comes with the following data types for storing a date or a date/time value in the database:

• DATE - format YYYY-MM-DD

• DATETIME - format: YYYY-MM-DD HH:MM:SS
• TIMESTAMP - format: YYYY-MM-DD HH:MM:SS
• YEAR - format YYYY or YY

SQL Server comes with the following data types for storing a date or a date/time value in the
database:

• DATE - format YYYY-MM-DD

• DATETIME - format: YYYY-MM-DD HH:MM:SS
• SMALLDATETIME - format: YYYY-MM-DD HH:MM:SS
• TIMESTAMP - format: a unique number

Note: The date types are chosen for a column when you create a new table in your database!

For an overview of all data types available, go to our complete Data Types reference.

SQL Working with Dates

You can compare two dates easily if there is no time component involved!

Assume we have the following "Orders" table:

OrderId ProductName OrderDate

1 Geitost 2008-11-11
2 Camembert Pierrot 2008-11-09
3 Mozzarella di Giovanni 2008-11-11
4 Mascarpone Fabioli 2008-10-29

Now we want to select the records with an OrderDate of "2008-11-11" from the table above.

We use the following SELECT statement:

SELECT * FROM Orders WHERE OrderDate='2008-11-11'

The result-set will look like this:

OrderId ProductName OrderDate

1 Geitost 2008-11-11
3 Mozzarella di Giovanni 2008-11-11

Now, assume that the "Orders" table looks like this (notice the time component in the "OrderDate"
column):

OrderId ProductName OrderDate

1 Geitost 2008-11-11 13:23:44
2 Camembert Pierrot 2008-11-09 15:45:21
3 Mozzarella di Giovanni 2008-11-11 11:12:01
4 Mascarpone Fabioli 2008-10-29 14:56:59

If we use the same SELECT statement as above:

SELECT * FROM Orders WHERE OrderDate='2008-11-11'

we will get no result! This is because the query is looking only for dates with no time portion.

Tip: If you want to keep your queries simple and easy to maintain, do not allow time components in
your dates!
 SQL NULL Values

NULL values represent missing unknown data.

By default, a table column can hold NULL values.

This chapter will explain the IS NULL and IS NOT NULL operators.

SQL NULL Values

If a column in a table is optional, we can insert a new record or update an existing record without
adding a value to this column. This means that the field will be saved with a NULL value.

NULL values are treated differently from other values.

NULL is used as a placeholder for unknown or inapplicable values.

Note: It is not possible to compare NULL and 0; they are not equivalent.

SQL Working with NULL Values

Look at the following "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Stavanger

Suppose that the "Address" column in the "Persons" table is optional. This means that if we insert a
record with no value for the "Address" column, the "Address" column will be saved with a NULL
value.

How can we test for NULL values?

It is not possible to test for NULL values with comparison operators, such as =, <, or <>.
We will have to use the IS NULL and IS NOT NULL operators instead.

SQL IS NULL
How do we select only the records with NULL values in the "Address" column?

We will have to use the IS NULL operator:

SELECT LastName,FirstName,Address FROM Persons

WHERE Address IS NULL

The result-set will look like this:

LastName FirstName Address

Hansen Ola
Pettersen Kari

Tip: Always use IS NULL to look for NULL values.

SQL IS NOT NULL

How do we select only the records with no NULL values in the "Address" column?

We will have to use the IS NOT NULL operator:

SELECT LastName,FirstName,Address FROM Persons

WHERE Address IS NOT NULL

The result-set will look like this:

LastName FirstName Address

Svendson Tove Borgvn 23

In the next chapter we will look at the ISNULL(), NVL(), IFNULL() and COALESCE() functions.
 SQL NULL Functions

SQL ISNULL(), NVL(), IFNULL() and COALESCE() Functions

Look at the following "Products" table:

P_Id ProductName UnitPrice UnitsInStock UnitsOnOrder

1 Jarlsberg 10.45 16 15
2 Mascarpone 32.56 23
3 Gorgonzola 15.67 9 20

Suppose that the "UnitsOnOrder" column is optional, and may contain NULL values.

We have the following SELECT statement:

SELECT ProductName,UnitPrice*(UnitsInStock+UnitsOnOrder)
FROM Products

In the example above, if any of the "UnitsOnOrder" values are NULL, the result is NULL.

Microsoft's ISNULL() function is used to specify how we want to treat NULL values.

The NVL(), IFNULL(), and COALESCE() functions can also be used to achieve the same result.

In this case we want NULL values to be zero.

Below, if "UnitsOnOrder" is NULL it will not harm the calculation, because ISNULL() returns a zero if
the value is NULL:

SQL Server / MS Access

SELECT ProductName,UnitPrice*(UnitsInStock+ISNULL(UnitsOnOrder,0))
FROM Products

Oracle

Oracle does not have an ISNULL() function. However, we can use the NVL() function to achieve the
same result:
SELECT ProductName,UnitPrice*(UnitsInStock+NVL(UnitsOnOrder,0))
FROM Products

MySQL

MySQL does have an ISNULL() function. However, it works a little bit different from Microsoft's
ISNULL() function.

In MySQL we can use the IFNULL() function, like this:

SELECT ProductName,UnitPrice*(UnitsInStock+IFNULL(UnitsOnOrder,0))
FROM Products

or we can use the COALESCE() function, like this:

SELECT ProductName,UnitPrice*(UnitsInStock+COALESCE(UnitsOnOrder,0))
FROM Products
 SQL Data Types

Data types and ranges for Microsoft Access, MySQL and SQL Server.

Microsoft Access Data Types

Data type Description Storage
Text Use for text or combinations of text and numbers. 255 characters
maximum
Memo Memo is used for larger amounts of text. Stores up to 65,536
characters. Note: You cannot sort a memo field. However, they are
searchable
Byte Allows whole numbers from 0 to 255 1 byte
Integer Allows whole numbers between -32,768 and 32,767 2 bytes
Long Allows whole numbers between -2,147,483,648 and 2,147,483,647 4 bytes
Single Single precision floating-point. Will handle most decimals 4 bytes
Double Double precision floating-point. Will handle most decimals 8 bytes
Currency Use for currency. Holds up to 15 digits of whole dollars, plus 4 decimal 8 bytes
places. Tip: You can choose which country's currency to use
AutoNumber AutoNumber fields automatically give each record its own number, 4 bytes
usually starting at 1
Date/Time Use for dates and times 8 bytes
Yes/No A logical field can be displayed as Yes/No, True/False, or On/Off. In 1 bit
code, use the constants True and False (equivalent to -1 and 0). Note:
Null values are not allowed in Yes/No fields
Ole Object Can store pictures, audio, video, or other BLOBs (Binary Large up to 1GB
OBjects)
Hyperlink Contain links to other files, including web pages
Lookup Wizard Let you type a list of options, which can then be chosen from a drop- 4 bytes
down list

MySQL Data Types

In MySQL there are three main types : text, number, and Date/Time types.

Text types:
Data type Description
CHAR(size) Holds a fixed length string (can contain letters, numbers, and special
characters). The fixed size is specified in parenthesis. Can store up to 255
characters
VARCHAR(size) Holds a variable length string (can contain letters, numbers, and special
characters). The maximum size is specified in parenthesis. Can store up to 255
characters. Note: If you put a greater value than 255 it will be converted to a
TEXT type
TINYTEXT Holds a string with a maximum length of 255 characters
TEXT Holds a string with a maximum length of 65,535 characters
BLOB For BLOBs (Binary Large OBjects). Holds up to 65,535 bytes of data
MEDIUMTEXT Holds a string with a maximum length of 16,777,215 characters
MEDIUMBLOB For BLOBs (Binary Large OBjects). Holds up to 16,777,215 bytes of data
LONGTEXT Holds a string with a maximum length of 4,294,967,295 characters
LONGBLOB For BLOBs (Binary Large OBjects). Holds up to 4,294,967,295 bytes of data
ENUM(x,y,z,etc.) Let you enter a list of possible values. You can list up to 65535 values in an
ENUM list. If a value is inserted that is not in the list, a blank value will be
inserted.

Note: The values are sorted in the order you enter them.

You enter the possible values in this format: ENUM('X','Y','Z')

SET Similar to ENUM except that SET may contain up to 64 list items and can store
more than one choice

Number types:

Data type Description

TINYINT(size) -128 to 127 normal. 0 to 255 UNSIGNED*. The maximum number of digits may
be specified in parenthesis
SMALLINT(size) -32768 to 32767 normal. 0 to 65535 UNSIGNED*. The maximum number of
digits may be specified in parenthesis
MEDIUMINT(size) -8388608 to 8388607 normal. 0 to 16777215 UNSIGNED*. The maximum
number of digits may be specified in parenthesis
INT(size) -2147483648 to 2147483647 normal. 0 to 4294967295 UNSIGNED*. The
maximum number of digits may be specified in parenthesis
BIGINT(size) -9223372036854775808 to 9223372036854775807 normal. 0 to
18446744073709551615 UNSIGNED*. The maximum number of digits may be
specified in parenthesis
FLOAT(size,d) A small number with a floating decimal point. The maximum number of digits
may be specified in the size parameter. The maximum number of digits to the
right of the decimal point is specified in the d parameter
DOUBLE(size,d) A large number with a floating decimal point. The maximum number of digits
may be specified in the size parameter. The maximum number of digits to the
right of the decimal point is specified in the d parameter
DECIMAL(size,d) A DOUBLE stored as a string , allowing for a fixed decimal point. The maximum
number of digits may be specified in the size parameter. The maximum number
of digits to the right of the decimal point is specified in the d parameter
*The integer types have an extra option called UNSIGNED. Normally, the integer goes from an
negative to positive value. Adding the UNSIGNED attribute will move that range up so it starts at
zero instead of a negative number.

Date types:

Data type Description

DATE() A date. Format: YYYY-MM-DD

Note: The supported range is from '1000-01-01' to '9999-12-31'

DATETIME() *A date and time combination. Format: YYYY-MM-DD HH:MM:SS

Note: The supported range is from '1000-01-01 00:00:00' to '9999-12-31

23:59:59'
TIMESTAMP() *A timestamp. TIMESTAMP values are stored as the number of seconds since the
Unix epoch ('1970-01-01 00:00:00' UTC). Format: YYYY-MM-DD HH:MM:SS

Note: The supported range is from '1970-01-01 00:00:01' UTC to '2038-01-09

03:14:07' UTC
TIME() A time. Format: HH:MM:SS

Note: The supported range is from '-838:59:59' to '838:59:59'

YEAR() A year in two-digit or four-digit format.

Note: Values allowed in four-digit format: 1901 to 2155. Values allowed in two-
digit format: 70 to 69, representing years from 1970 to 2069

*Even if DATETIME and TIMESTAMP return the same format, they work very differently. In an
INSERT or UPDATE query, the TIMESTAMP automatically set itself to the current date and time.
TIMESTAMP also accepts various formats, like YYYYMMDDHHMMSS, YYMMDDHHMMSS, YYYYMMDD,
or YYMMDD.

SQL Server Data Types

Character strings:

Data type Description Storage

char(n) Fixed-length character string. Maximum 8,000 characters n
varchar(n) Variable-length character string. Maximum 8,000 characters
varchar(max) Variable-length character string. Maximum 1,073,741,824 characters
text Variable-length character string. Maximum 2GB of text data

Unicode strings:

Data type Description Storage

nchar(n) Fixed-length Unicode data. Maximum 4,000 characters
nvarchar(n) Variable-length Unicode data. Maximum 4,000 characters
nvarchar(max) Variable-length Unicode data. Maximum 536,870,912 characters
ntext Variable-length Unicode data. Maximum 2GB of text data

Binary types:

Data type Description Storage

bit Allows 0, 1, or NULL
binary(n) Fixed-length binary data. Maximum 8,000 bytes
varbinary(n) Variable-length binary data. Maximum 8,000 bytes
varbinary(max) Variable-length binary data. Maximum 2GB
image Variable-length binary data. Maximum 2GB

Number types:

Data type Description Storage

tinyint Allows whole numbers from 0 to 255 1 byte
smallint Allows whole numbers between -32,768 and 32,767 2 bytes
int Allows whole numbers between -2,147,483,648 and 2,147,483,647 4 bytes
bigint Allows whole numbers between -9,223,372,036,854,775,808 and 8 bytes
9,223,372,036,854,775,807
decimal(p,s) Fixed precision and scale numbers. 5-17
bytes
Allows numbers from -10^38 +1 to 10^38 –1.

The p parameter indicates the maximum total number of digits that

can be stored (both to the left and to the right of the decimal point). p
must be a value from 1 to 38. Default is 18.

The s parameter indicates the maximum number of digits stored to the

right of the decimal point. s must be a value from 0 to p. Default value
is 0
numeric(p,s) Fixed precision and scale numbers. 5-17
bytes
Allows numbers from -10^38 +1 to 10^38 –1.

The p parameter indicates the maximum total number of digits that

can be stored (both to the left and to the right of the decimal point). p
must be a value from 1 to 38. Default is 18.

The s parameter indicates the maximum number of digits stored to the

right of the decimal point. s must be a value from 0 to p. Default value
is 0
smallmoney Monetary data from -214,748.3648 to 214,748.3647 4 bytes
money Monetary data from -922,337,203,685,477.5808 to 8 bytes
922,337,203,685,477.5807
float(n) Floating precision number data from -1.79E + 308 to 1.79E + 308. 4 or 8
bytes
The n parameter indicates whether the field should hold 4 or 8 bytes.
float(24) holds a 4-byte field and float(53) holds an 8-byte field.
Default value of n is 53.
real Floating precision number data from -3.40E + 38 to 3.40E + 38 4 bytes

Date types:

Data type Description Storage

datetime From January 1, 1753 to December 31, 9999 with an accuracy of 3.33 8 bytes
milliseconds
datetime2 From January 1, 0001 to December 31, 9999 with an accuracy of 100 6-8 bytes
nanoseconds
smalldatetime From January 1, 1900 to June 6, 2079 with an accuracy of 1 minute 4 bytes
date Store a date only. From January 1, 0001 to December 31, 9999 3 bytes
time Store a time only to an accuracy of 100 nanoseconds 3-5 bytes
datetimeoffset The same as datetime2 with the addition of a time zone offset 8-10
bytes
timestamp Stores a unique number that gets updated every time a row gets
created or modified. The timestamp value is based upon an internal
clock and does not correspond to real time. Each table may have only
one timestamp variable

Other data types:

Data type Description

sql_variant Stores up to 8,000 bytes of data of various data types, except text, ntext, and
timestamp
uniqueidentifier Stores a globally unique identifier (GUID)
xml Stores XML formatted data. Maximum 2GB
cursor Stores a reference to a cursor used for database operations
table Stores a result-set for later processing
 SQL Functions

SQL has many built-in functions for performing calculations on data.

SQL Aggregate Functions

SQL aggregate functions return a single value, calculated from values in a column.

Useful aggregate functions:

• AVG() - Returns the average value

• COUNT() - Returns the number of rows
• FIRST() - Returns the first value
• LAST() - Returns the last value
• MAX() - Returns the largest value
• MIN() - Returns the smallest value
• SUM() - Returns the sum

SQL Scalar functions

SQL scalar functions return a single value, based on the input value.

Useful scalar functions:

• UCASE() - Converts a field to upper case

• LCASE() - Converts a field to lower case
• MID() - Extract characters from a text field
• LEN() - Returns the length of a text field
• ROUND() - Rounds a numeric field to the number of decimals specified
• NOW() - Returns the current system date and time
• FORMAT() - Formats how a field is to be displayed

Tip: The aggregate functions and the scalar functions will be explained in details in the next
chapters.
 SQL AVG() Function

The AVG() Function

The AVG() function returns the average value of a numeric column.

SQL AVG() Syntax

SELECT AVG(column_name) FROM table_name

SQL AVG() Example

We have the following "Orders" table:

O_Id OrderDate OrderPrice Customer

1 2008/11/12 1000 Hansen
2 2008/10/23 1600 Nilsen
3 2008/09/02 700 Hansen
4 2008/09/03 300 Hansen
5 2008/08/30 2000 Jensen
6 2008/10/04 100 Nilsen
Now we want to find the average value of the "OrderPrice" fields.

We use the following SQL statement:

SELECT AVG(OrderPrice) AS OrderAverage FROM Orders

The result-set will look like this:

OrderAverage
950
Now we want to find the customers that have
an OrderPrice value higher than the average OrderPrice value.

We use the following SQL statement:

SELECT Customer FROM Orders

WHERE OrderPrice>(SELECT AVG(OrderPrice) FROM Orders)
The result-set will look like this:
Customer
Hansen
Nilsen
Jensen
 SQL COUNT() Function

The COUNT() function returns the number of rows that matches a specified criteria.

SQL COUNT(column_name) Syntax

The COUNT(column_name) function returns the number of values (NULL values will not be counted)
of the specified column:

SELECT COUNT(column_name) FROM table_name

SQL COUNT(*) Syntax

The COUNT(*) function returns the number of records in a table:

SELECT COUNT(*) FROM table_name

SQL COUNT(DISTINCT column_name) Syntax

The COUNT(DISTINCT column_name) function returns the number of distinct values of the specified
column:

SELECT COUNT(DISTINCT column_name) FROM table_name

Note: COUNT(DISTINCT) works with ORACLE and Microsoft SQL Server, but not with Microsoft
Access.

SQL COUNT(column_name) Example

We have the following "Orders" table:

O_Id OrderDate OrderPrice Customer

1 2008/11/12 1000 Hansen
2 2008/10/23 1600 Nilsen
3 2008/09/02 700 Hansen
4 2008/09/03 300 Hansen
5 2008/08/30 2000 Jensen
6 2008/10/04 100 Nilsen

Now we want to count the number of orders from "Customer Nilsen".

We use the following SQL statement:

SELECT COUNT(Customer) AS CustomerNilsen FROM Orders

WHERE Customer='Nilsen'

The result of the SQL statement above will be 2, because the customer Nilsen has made 2 orders in
total:

CustomerNilsen
2

SQL COUNT(*) Example

If we omit the WHERE clause, like this:

SELECT COUNT(*) AS NumberOfOrders FROM Orders

The result-set will look like this:

NumberOfOrders
6

which is the total number of rows in the table.

SQL COUNT(DISTINCT column_name) Example

Now we want to count the number of unique customers in the "Orders" table.

We use the following SQL statement:

SELECT COUNT(DISTINCT Customer) AS NumberOfCustomers FROM Orders

The result-set will look like this:

NumberOfCustomers
3

which is the number of unique customers (Hansen, Nilsen, and Jensen) in the "Orders" table.

SQL FIRST() Function

 The FIRST() Function
The FIRST() function returns the first value of the selected column.

SQL FIRST() Syntax

SELECT FIRST(column_name) FROM table_name

SQL FIRST() Example

We have the following "Orders" table:

O_Id OrderDate OrderPrice Customer

1 2008/11/12 1000 Hansen
2 2008/10/23 1600 Nilsen
3 2008/09/02 700 Hansen
4 2008/09/03 300 Hansen
5 2008/08/30 2000 Jensen
6 2008/10/04 100 Nilsen
Now we want to find the first value of the "OrderPrice" column.

We use the following SQL statement:

SELECT FIRST(OrderPrice) AS FirstOrderPrice FROM Orders

The result-set will look like this:

FirstOrderPrice
1000

SQL LAST() Function

The LAST() Function

The LAST() function returns the last value of the selected column.
SQL LAST() Syntax
SELECT LAST(column_name) FROM table_name

SQL LAST() Example

We have the following "Orders" table:

O_Id OrderDate OrderPrice Customer

1 2008/11/12 1000 Hansen
2 2008/10/23 1600 Nilsen
3 2008/09/02 700 Hansen
4 2008/09/03 300 Hansen
5 2008/08/30 2000 Jensen
6 2008/10/04 100 Nilsen

Now we want to find the last value of the "OrderPrice" column.

We use the following SQL statement:

SELECT LAST(OrderPrice) AS LastOrderPrice FROM Orders

The result-set will look like this:

LastOrderPrice
100

SQL MAX() Function

The MAX() Function

The MAX() function returns the largest value of the selected column.

SQL MAX() Syntax

 SELECT MAX(column_name) FROM table_name

SQL MAX() Example

We have the following "Orders" table:

O_Id OrderDate OrderPrice Customer

1 2008/11/12 1000 Hansen
2 2008/10/23 1600 Nilsen
3 2008/09/02 700 Hansen
4 2008/09/03 300 Hansen
5 2008/08/30 2000 Jensen
6 2008/10/04 100 Nilsen
Now we want to find the largest value of the "OrderPrice" column.

We use the following SQL statement:

SELECT MAX(OrderPrice) AS LargestOrderPrice FROM Orders

The result-set will look like this:

LargestOrderPrice
2000

SQL MIN() Function

The MIN() Function

The MIN() function returns the smallest value of the selected column.

SQL MIN() Syntax

SELECT MIN(column_name) FROM table_name
SQL MIN() Example
We have the following "Orders" table:

O_Id OrderDate OrderPrice Customer

1 2008/11/12 1000 Hansen
2 2008/10/23 1600 Nilsen
3 2008/09/02 700 Hansen
4 2008/09/03 300 Hansen
5 2008/08/30 2000 Jensen
6 2008/10/04 100 Nilsen

Now we want to find the smallest value of the "OrderPrice" column.

We use the following SQL statement:

SELECT MIN(OrderPrice) AS SmallestOrderPrice FROM Orders

The result-set will look like this:

SmallestOrderPrice
100

SQL SUM() Function

The SUM() Function

The SUM() function returns the total sum of a numeric column.

SQL SUM() Syntax

SELECT SUM(column_name) FROM table_name
SQL SUM() Example
We have the following "Orders" table:

O_Id OrderDate OrderPrice Customer

1 2008/11/12 1000 Hansen
2 2008/10/23 1600 Nilsen
3 2008/09/02 700 Hansen
4 2008/09/03 300 Hansen
5 2008/08/30 2000 Jensen
6 2008/10/04 100 Nilsen

Now we want to find the sum of all "OrderPrice" fields".

We use the following SQL statement:

SELECT SUM(OrderPrice) AS OrderTotal FROM Orders

The result-set will look like this:

OrderTotal
5700

SQL GROUP BY Statement

Aggregate functions often need an added GROUP BY statement.

The GROUP BY Statement

The GROUP BY statement is used in conjunction with the aggregate functions to group the result-set
by one or more columns.

SQL GROUP BY Syntax

SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name

SQL GROUP BY Example

We have the following "Orders" table:

O_Id OrderDate OrderPrice Customer

1 2008/11/12 1000 Hansen
2 2008/10/23 1600 Nilsen
3 2008/09/02 700 Hansen
4 2008/09/03 300 Hansen
5 2008/08/30 2000 Jensen
6 2008/10/04 100 Nilsen

Now we want to find the total sum (total order) of each customer.

We will have to use the GROUP BY statement to group the customers.

We use the following SQL statement:

SELECT Customer,SUM(OrderPrice) FROM Orders

GROUP BY Customer

The result-set will look like this:

Customer SUM(OrderPrice)
Hansen 2000
Nilsen 1700
Jensen 2000

Nice! Isn't it? :)

Let's see what happens if we omit the GROUP BY statement:

SELECT Customer,SUM(OrderPrice) FROM Orders

The result-set will look like this:

Customer SUM(OrderPrice)
Hansen 5700
Nilsen 5700
Hansen 5700
Hansen 5700
Jensen 5700
Nilsen 5700

The result-set above is not what we wanted.

Explanation of why the above SELECT statement cannot be used: The SELECT statement
above has two columns specified (Customer and SUM(OrderPrice). The "SUM(OrderPrice)" returns a
single value (that is the total sum of the "OrderPrice" column), while "Customer" returns 6 values
(one value for each row in the "Orders" table). This will therefore not give us the correct result.
However, you have seen that the GROUP BY statement solves this problem.

GROUP BY More Than One Column

We can also use the GROUP BY statement on more than one column, like this:

SELECT Customer,OrderDate,SUM(OrderPrice) FROM Orders

GROUP BY Customer,OrderDate

SQL HAVING Clause

The HAVING Clause

The HAVING clause was added to SQL because the WHERE keyword could not be used with
aggregate functions.

SQL HAVING Syntax

SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
HAVING aggregate_function(column_name) operator value

SQL HAVING Example

We have the following "Orders" table:

O_Id OrderDate OrderPrice Customer

1 2008/11/12 1000 Hansen
2 2008/10/23 1600 Nilsen
3 2008/09/02 700 Hansen
4 2008/09/03 300 Hansen
5 2008/08/30 2000 Jensen
6 2008/10/04 100 Nilsen
Now we want to find if any of the customers have a total order of less than 2000.

We use the following SQL statement:

SELECT Customer,SUM(OrderPrice) FROM Orders

GROUP BY Customer
HAVING SUM(OrderPrice)<2000
The result-set will look like this:

Customer SUM(OrderPrice)
Nilsen 1700
Now we want to find if the customers "Hansen" or
"Jensen" have a total order of more than 1500.

We add an ordinary WHERE clause to the SQL statement:

SELECT Customer,SUM(OrderPrice) FROM Orders

WHERE Customer='Hansen' OR Customer='Jensen'
GROUP BY Customer
HAVING SUM(OrderPrice)>1500
The result-set will look like this:

Customer SUM(OrderPrice)
Hansen 2000
Jensen 2000
 SQL UCASE() Function

The UCASE() Function

The UCASE() function converts the value of a field to uppercase.

SQL UCASE() Syntax

SELECT UCASE(column_name) FROM table_name

Syntax for SQL Server

SELECT UPPER(column_name) FROM table_name

SQL UCASE() Example

We have the following "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger
Now we want to select the content of the "LastName" and "FirstName" columns above, and convert
the "LastName" column to uppercase.

We use the following SELECT statement:

SELECT UCASE(LastName) as LastName,FirstName FROM Persons

The result-set will look like this:

LastName FirstName
HANSEN Ola
SVENDSON Tove
PETTERSEN Kari

SQL LCASE() Function

The LCASE() Function

The LCASE() function converts the value of a field to lowercase.

SQL LCASE() Syntax

SELECT LCASE(column_name) FROM table_name

Syntax for SQL Server

SELECT LOWER(column_name) FROM table_name

SQL LCASE() Example

We have the following "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger
Now we want to select the content of the "LastName" and "FirstName" columns above, and convert
the "LastName" column to lowercase.

We use the following SELECT statement:

SELECT LCASE(LastName) as LastName,FirstName FROM Persons

The result-set will look like this:

LastName FirstName
hansen Ola
svendson Tove
pettersen Kari

SQL MID() Function

The MID() Function

The MID() function is used to extract characters from a text field.

SQL MID() Syntax

SELECT MID(column_name,start[,length]) FROM table_name

Parameter Description
column_name Required. The field to extract characters from
start Required. Specifies the starting position (starts at 1)
length Optional. The number of characters to return. If omitted, the MID() function
returns the rest of the text

SQL MID() Example

We have the following "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger
Now we want to extract the first four characters of the "City" column above.

We use the following SELECT statement:

SELECT MID(City,1,4) as SmallCity FROM Persons

The result-set will look like this:

SmallCity
Sand
Sand
Stav

SQL LEN() Function

The LEN() Function

The LEN() function returns the length of the value in a text field.

SQL LEN() Syntax

SELECT LEN(column_name) FROM table_name

SQL LEN() Example

We have the following "Persons" table:

P_Id LastName FirstName Address City

1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

Now we want to select the length of the values in the "Address" column above.

We use the following SELECT statement:

SELECT LEN(Address) as LengthOfAddress FROM Persons

The result-set will look like this:

LengthOfAddress
12
9
9

SQL ROUND() Function

The ROUND() Function

The ROUND() function is used to round a numeric field to the number of decimals specified.

SQL ROUND() Syntax

SELECT ROUND(column_name,decimals) FROM table_name

Parameter Description
column_name Required. The field to round.
decimals Required. Specifies the number of decimals to be returned.

SQL ROUND() Example

We have the following "Products" table:

Prod_Id ProductName Unit UnitPrice

1 Jarlsberg 1000 g 10.45
2 Mascarpone 1000 g 32.56
3 Gorgonzola 1000 g 15.67

Now we want to display the product name and the price rounded to the nearest integer.

We use the following SELECT statement:

SELECT ProductName, ROUND(UnitPrice,0) as UnitPrice FROM Products

The result-set will look like this:

ProductName UnitPrice
Jarlsberg 10
Mascarpone 33
Gorgonzola 16

SQL NOW() Function

The NOW() Function

The NOW() function returns the current system date and time.

SQL NOW() Syntax

SELECT NOW() FROM table_name

SQL NOW() Example

We have the following "Products" table:

Prod_Id ProductName Unit UnitPrice

1 Jarlsberg 1000 g 10.45
2 Mascarpone 1000 g 32.56
3 Gorgonzola 1000 g 15.67

Now we want to display the products and prices per today's date.

We use the following SELECT statement:

SELECT ProductName, UnitPrice, Now() as PerDate FROM Products

The result-set will look like this:

ProductName UnitPrice PerDate

Jarlsberg 10.45 10/7/2008 11:25:02 AM
Mascarpone 32.56 10/7/2008 11:25:02 AM
Gorgonzola 15.67 10/7/2008 11:25:02 AM

SQL FORMAT() Function

The FORMAT() Function

The FORMAT() function is used to format how a field is to be displayed.

SQL FORMAT() Syntax

SELECT FORMAT(column_name,format) FROM table_name

Parameter Description
column_name Required. The field to be formatted.
format Required. Specifies the format.

SQL FORMAT() Example

We have the following "Products" table:

Prod_Id ProductName Unit UnitPrice

1 Jarlsberg 1000 g 10.45
2 Mascarpone 1000 g 32.56
3 Gorgonzola 1000 g 15.67

Now we want to display the products and prices per today's date (with today's date displayed in the
following format "YYYY-MM-DD").

We use the following SELECT statement:

SELECT ProductName, UnitPrice, FORMAT(Now(),'YYYY-MM-DD') as PerDate

FROM Products
The result-set will look like this:

ProductName UnitPrice PerDate

Jarlsberg 10.45 2008-10-07
Mascarpone 32.56 2008-10-07
Gorgonzola 15.67 2008-10-07

SQL Quick Reference From W3Schools

SQL Statement Syntax

AND / OR SELECT column_name(s)
FROM table_name
WHERE condition
AND|OR condition
ALTER TABLE ALTER TABLE table_name
ADD column_name datatype

or

ALTER TABLE table_name

DROP COLUMN column_name
AS (alias) SELECT column_name AS column_alias
FROM table_name

or

SELECT column_name
FROM table_name AS table_alias
BETWEEN SELECT column_name(s)
FROM table_name
WHERE column_name
BETWEEN value1 AND value2
CREATE DATABASE CREATE DATABASE database_name
CREATE TABLE CREATE TABLE table_name
(
column_name1 data_type,
column_name2 data_type,
column_name2 data_type,
...
)
CREATE INDEX CREATE INDEX index_name
ON table_name (column_name)

or

CREATE UNIQUE INDEX index_name

 ON table_name (column_name)
CREATE VIEW CREATE VIEW view_name AS
SELECT column_name(s)
FROM table_name
WHERE condition
DELETE DELETE FROM table_name
WHERE some_column=some_value

or

DELETE FROM table_name

(Note: Deletes the entire table!!)

DELETE * FROM table_name

(Note: Deletes the entire table!!)
DROP DATABASE DROP DATABASE database_name
DROP INDEX DROP INDEX table_name.index_name (SQL Server)
DROP INDEX index_name ON table_name (MS Access)
DROP INDEX index_name (DB2/Oracle)
ALTER TABLE table_name
DROP INDEX index_name (MySQL)
DROP TABLE DROP TABLE table_name
GROUP BY SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
HAVING SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
HAVING aggregate_function(column_name) operator value
IN SELECT column_name(s)
FROM table_name
WHERE column_name
IN (value1,value2,..)
INSERT INTO INSERT INTO table_name
VALUES (value1, value2, value3,....)

or

INSERT INTO table_name

(column1, column2, column3,...)
VALUES (value1, value2, value3,....)
INNER JOIN SELECT column_name(s)
FROM table_name1
INNER JOIN table_name2
ON table_name1.column_name=table_name2.column_name
LEFT JOIN SELECT column_name(s)
FROM table_name1
LEFT JOIN table_name2
ON table_name1.column_name=table_name2.column_name
RIGHT JOIN SELECT column_name(s)
FROM table_name1
 RIGHT JOIN table_name2
ON table_name1.column_name=table_name2.column_name
FULL JOIN SELECT column_name(s)
FROM table_name1
FULL JOIN table_name2
ON table_name1.column_name=table_name2.column_name
LIKE SELECT column_name(s)
FROM table_name
WHERE column_name LIKE pattern
ORDER BY SELECT column_name(s)
FROM table_name
ORDER BY column_name [ASC|DESC]
SELECT SELECT column_name(s)
FROM table_name
SELECT * SELECT *
FROM table_name
SELECT DISTINCT SELECT DISTINCT column_name(s)
FROM table_name
SELECT INTO SELECT *
INTO new_table_name [IN externaldatabase]
FROM old_table_name

or

SELECT column_name(s)
INTO new_table_name [IN externaldatabase]
FROM old_table_name
SELECT TOP SELECT TOP number|percent column_name(s)
FROM table_name
TRUNCATE TABLE TRUNCATE TABLE table_name
UNION SELECT column_name(s) FROM table_name1
UNION
SELECT column_name(s) FROM table_name2
UNION ALL SELECT column_name(s) FROM table_name1
UNION ALL
SELECT column_name(s) FROM table_name2
UPDATE UPDATE table_name
SET column1=value, column2=value,...
WHERE some_column=some_value
WHERE SELECT column_name(s)
FROM table_name
WHERE column_name operator value

Source : http://www.w3schools.com/sql/sql_quickref.asp