SQL Tutorial
SQL is a standard language for storing, manipulating and retrieving data in databases.
Our SQL tutorial will teach you how to use SQL in: MySQL, SQL Server, MS Access, Oracle, Sybase,
Informix, Postgres, and other database systems.
What is SQL?
SQL stands for Structured Query Language.
SQL lets you access and manipulate databases.
SQL became a standard of the American National Standards Institute (ANSI)in 1986, and of the International
Organization for Standardization (ISO) in1987.
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/ISO standard, there are 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!
RDBMS
RDBMS stands for Relational Database Management System.
RDBMS is the basis for SQL, and for all modern database systems such as 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 collection of related data entries and
it consists of columns and rows.
Every table is broken up into smaller entities called fields. The fields in the Customers table consist of
CustomerID, CustomerName, ContactName, Address, City, PostalCode and Country. A field is a column in a
table that is designed to maintain specific information about every record in the table.
A record, also called a row, is each individual entry that exists in a table. For example, there are 91 records in
the above Customers table. A record is a horizontal entity in a table.
A column is a vertical entity in a table that contains all information associated with a specific field in a table.
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.
SQL Statements
Most of the actions you need to perform on a database are done with SQL statements.
The following SQL statement selects all the records in the "Customers" table:
SELECT * FROM Customers;
Keep in Mind That...
SQL keywords are NOT case sensitive: select is the same as SELECT
In this tutorial we will write all SQL keywords in upper-case
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.
In this tutorial, we will use semicolon at the end of each SQL statement.
Some of The Most Important SQL Commands
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
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
The SQL SELECT Statement
The SELECT statement is used to select data from a database.
SELECT Syntax
SELECT column1, column2, ...
FROM table_name;
Here, column1, column2, ... are the field names of the table you want to select data from. If you want to select
all the fields available in the table, use the following syntax:
SELECT * FROM table_name;
The SQL SELECT DISTINCT Statement
The SELECT DISTINCT statement is used to return only distinct (different) values.
SELECT DISTINCT Syntax
SELECT DISTINCT column1, column2, ...
FROM table_name;
The SQL WHERE Clause
The WHERE clause is used to filter records.
The WHERE clause is used to extract only those records that fulfill a specified condition.
WHERE Syntax
SELECT column1, column2, ...
FROM table_name
WHERE condition;
Note: The WHERE clause is not only used in SELECT statement, it is also used in UPDATE, DELETE
statement, etc.!that
WHERE Clause Example
The following SQL statement selects all the customers from the country "Mexico", in the "Customers" table:
SELECT * FROM Customers
WHERE Country='Mexico';
Text Fields vs. Numeric Fields
SQL requires single quotes around text values (most database systems will also allow double quotes).
However, numeric fields should not be enclosed in quotes:
SELECT * FROM Customers
WHERE CustomerID=1;
Operators in The WHERE Clause
The following operators can be used in the WHERE clause:
Operator Description
= Equal
> Greater than
< Less than
>= Greater than or equal
<= Less than or equal
Not equal. Note: In some versions of SQL this
<>
operator may be written as !=
BETWEEN Between a certain range
LIKE Search for a pattern
IN To specify multiple possible values for a column
