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Basics of the SELECT Statement www.thecodexpert.com
In a relational database, data is stored in tables. An example table would relate Social Security Number, Name, and
Address:
EmployeeAddressTable
SSN FirstName LastName Address City State
512687458 Joe Smith 83 First Street Howard Ohio
758420012 Mary Scott 842 Vine Ave. Losantiville Ohio
102254896 Sam Jones 33 Elm St. Paris New York
876512563 Sarah Ackerman 440 U.S. 110 Upton Michigan
Query 1 let’s say you want to see the address of each employee. Use the SELECT statement, like so:
Conditional Selection
To further discuss the SELECT statement, let's look at a new example table (for hypothetical purposes only):
EmployeeStatisticsTable
EmployeeIDNo Salary Benefits Position
010 75000 15000 Manager
105 65000 15000 Manager
152 60000 15000 Manager
215 60000 12500 Manager
244 50000 12000 Staff
300 45000 10000 Staff
335 40000 10000 Staff
400 32000 7500 Entry-Level
441 28000 7500 Entry-Level
Relational Operators
There are six Relational Operators in SQL, and after introducing them, we'll see how they're used:
= Equal
< or != (see
Not Equal
manual)
< Less Than
> Greater Than
<= Less Than or Equal To
>= Greater Than or Equal To
The WHERE clause is used to specify that only certain rows of the table are displayed, based on the criteria described
in that WHERE clause. It is most easily understood by looking at a couple of examples.
Query 2 EMPLOYEEIDNO's of those making at or over $50,000
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More Complex Conditions: Compound Conditions / Logical Operators
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The AND operator joins two or more conditions, and displays a row only if that row's data satisfies ALL conditions
listed (i.e. all conditions hold true).
Query 3
To display all staff making over $40,000, use:
The OR operator joins two or more conditions, but returns a row if ANY of the conditions listed hold true.
Query 4
To see all those who make less than $40,000 or have less than $10,000 in benefits, listed together, use the following
query:
AND & OR can be combined, for example:
Query 5
Find all managers whose salary is greater than 60000 and benefits are greater than 12000
IN & BETWEEN
An easier method of using compound conditions uses IN or BETWEEN.
Query 6
For example, if you wanted to list all managers and staff:
Query 7
To list those making greater than or equal to $30,000, but less than or equal to $50,000, use:
Query 8
To list everyone not in this range, try:
Using LIKE
Query 9
Look at the EmployeeStatisticsTable, and say you wanted to see all people whose last names started with "S"; try:
Joins
In this section, we will only discuss inner joins, and equijoins, as in general, they are the most useful. For more
information, try the SQL links at the bottom of the page.
Good database design suggests that each table lists data only about a single entity, and detailed information can be
obtained in a relational database, by using additional tables, and by using a join.
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First, take a look at these example tables: www.thecodexpert.com
AntiqueOwners
OwnerID OwnerLastName OwnerFirstName
01 Jones Bill
02 Smith Bob
15 Lawson Patricia
21 Akins Jane
50 Fowler Sam
Orders
OwnerID ItemDesired
02 Table
02 Desk
21 Chair
15 Mirror
Antiques
SellerID BuyerID Item
01 50 Bed
02 15 Table
15 02 Chair
21 50 Mirror
50 01 Desk
01 21 Cabinet
02 21 Coffee Table
15 50 Chair
01 15 Jewelry Box
02 21 Pottery
21 02 Bookcase
50 01 Plant Stand
Keys
First, let's discuss the concept of keys. A primary key is a column or set of columns that uniquely identifies the rest of
the data in any given row. For example, in the AntiqueOwners table, the OwnerID column uniquely identifies that row.
This means two things: no two rows can have the same OwnerID, and, even if two owners have the same first and last
names, the OwnerID column ensures that the two owners will not be confused with each other, because the unique
OwnerID column will be used throughout the database to track the owners, rather than the names.
A foreign key is a column in a table where that column is a primary key of another table, which means that any data in a
foreign key column must have corresponding data in the other table where that column is the primary key. In DBMS-
speak, this correspondence is known as referential integrity. For example, in the Antiques table, both the BuyerID and
SellerID are foreign keys to the primary key of the AntiqueOwners table (OwnerID; for purposes of argument, one has
to be an Antique Owner before one can buy or sell any items), as, in both tables, the ID rows are used to identify the
owners or buyers and sellers, and that the OwnerID is the primary key of the AntiqueOwners table. In other words, all
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of this "ID" data is used to refer to the owners, www.thecodexpert.com
buyers, or sellers of antiques, themselves, without having to use the
actual names.
Performing a Join
The purpose of these keys is so that data can be related across tables, without having to repeat data in every table--this
is the power of relational databases. For example, you can find the names of those who bought a chair without having
to list the full name of the buyer in the Antiques table...you can get the name by relating those who bought a chair with
the names in the AntiqueOwners table through the use of the OwnerID, which relates the data in the two tables.
Query 10
To find the names of those who bought a chair, use the following query:
DISTINCT and Eliminating Duplicates
Let's say that you want to list the ID and names of only those people who have sold an antique. Obviously, you want a
list where each seller is only listed once--you don't want to know how many antiques a person sold, just the fact that
this person sold one (for counts, see the Aggregate Function section below). This means that you will need to tell SQL
to eliminate duplicate sales rows, and just list each person only once. To do this, use the DISTINCT keyword.
First, we will need an equijoin to the AntiqueOwners table to get the detail data of the person's LastName and
FirstName. However, keep in mind that since the SellerID column in the Antiques table is a foreign key to the
AntiqueOwners table, a seller will only be listed if there is a row in the AntiqueOwners table listing the ID and names.
We also want to eliminate multiple occurrences of the SellerID in our listing, so we use DISTINCT on the column
where the repeats may occur (however, it is generally not necessary to strictly put the Distinct in front of the column
name).
Miscellaneous SQL Statements
Aggregate Functions
I will discuss five important aggregate functions: SUM, AVG, MAX, MIN, and COUNT. They are called aggregate
functions because they summarize the results of a query, rather than listing all of the rows.
SUM () gives the total of all the rows, satisfying any conditions, of the given column, where the given column
is numeric.
AVG () gives the average of the given column.
MAX () gives the largest figure in the given column.
MIN () gives the smallest figure in the given column.
COUNT(*) gives the number of rows satisfying the conditions.
Looking at the tables at the top of the document, let's look at three examples:
SELECT SUM(SALARY), AVG(SALARY)
FROM EMPLOYEESTATISTICSTABLE;
This query shows the total of all salaries in the table, and the average salary of all of the entries in the table.
SELECT MIN(BENEFITS)
FROM EMPLOYEESTATISTICSTABLE
WHERE POSITION = 'Manager';
This query gives the smallest figure of the Benefits column, of the employees who are Managers, which is 12500.
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SELECT COUNT(*) www.thecodexpert.com
FROM EMPLOYEESTATISTICSTABLE
WHERE POSITION = 'Staff';
This query tells you how many employees have Staff status (3).
Views
In SQL, you might (check your DBA) have access to create views for yourself. What a view does is to allow you to
assign the results of a query to a new, personal table, that you can use in other queries, where this new table is given the
view name in your FROM clause. When you access a view, the query that is defined in your view creation statement is
performed (generally), and the results of that query look just like another table in the query that you wrote invoking the
view. For example, to create a view:
CREATE VIEW ANTVIEW AS SELECT ITEMDESIRED FROM ORDERS;
Now, write a query using this view as a table, where the table is just a listing of all Items Desired from the Orders table:
SELECT SELLERID
FROM ANTIQUES, ANTVIEW
WHERE ITEMDESIRED = ITEM;
This query shows all SellerID's from the Antiques table where the Item in that table happens to appear in the Antview
view, which is just all of the Items Desired in the Orders table. The listing is generated by going through the Antique
Items one-by-one until there's a match with the Antview view. Views can be used to restrict database access, as well as,
in this case, simplify a complex query.
Creating New Tables
All tables within a database must be created at some point in time...let's see how we would create the Orders table:
CREATE TABLE ORDERS
(OWNERID INTEGER NOT NULL,
ITEMDESIRED CHAR(40) NOT NULL);
This statement gives the table name and tells the DBMS about each column in the table. Please note that this statement
uses generic data types, and that the data types might be different, depending on what DBMS you are using. As usual,
check local listings. Some common generic data types are:
Char(x) - A column of characters, where x is a number designating the maximum number of characters allowed
(maximum length) in the column.
Integer - A column of whole numbers, positive or negative.
Decimal(x, y) - A column of decimal numbers, where x is the maximum length in digits of the decimal numbers
in this column, and y is the maximum number of digits allowed after the decimal point. The maximum (4,2)
number would be 99.99.
Date - A date column in a DBMS-specific format.
Logical - A column that can hold only two values: TRUE or FALSE.
One other note, the NOT NULL means that the column must have a value in each row. If NULL was used, that column
may be left empty in a given row.
Altering Tables
Let's add a column to the Antiques table to allow the entry of the price of a given Item (Parentheses optional):
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ALTER TABLE ANTIQUES ADD (PRICE DECIMAL(8,2) NULL);
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The data for this new column can be updated or inserted as shown later.
Adding Data
To insert rows into a table, do the following:
INSERT INTO ANTIQUES VALUES (21, 01, 'Ottoman', 200.00);
This inserts the data into the table, as a new row, column-by-column, in the pre-defined order. Instead, let's change the
order and leave Price blank:
INSERT INTO ANTIQUES (BUYERID, SELLERID, ITEM)
VALUES (01, 21, 'Ottoman');
Deleting Data
Let's delete this new row back out of the database:
DELETE FROM ANTIQUES
WHERE ITEM = 'Ottoman';
But if there is another row that contains 'Ottoman', that row will be deleted also. Let's delete all rows (one, in this case)
that contain the specific data we added before:
DELETE FROM ANTIQUES
WHERE ITEM = 'Ottoman' AND BUYERID = 01 AND SELLERID = 21;
Updating Data
Let's update a Price into a row that doesn't have a price listed yet:
UPDATE ANTIQUES SET PRICE = 500.00 WHERE ITEM = 'Chair';
This sets all Chair's Prices to 500.00. As shown above, more WHERE conditionals, using AND, must be used to limit
the updating to more specific rows. Also, additional columns may be set by separating equal statements with commas.
GROUP BY & HAVING
One special use of GROUP BY is to associate an aggregate function (especially COUNT; counting the number of rows
in each group) with groups of rows. If aggregate functions are included in the SELECT clause <select list>, GROUP
BY calculates a summary value for each group.
First, assume that the Antiques table has the Price column, and each row has a value for that column. We want to see
the price of the most expensive item bought by each owner. We have to tell SQL to group each owner's purchases, and
tell us the maximum purchase price:
SELECT BUYERID, MAX(PRICE)
FROM ANTIQUES
GROUP BY BUYERID;
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Now, say we only want to see the maximum purchase price if the purchase is over $1000, so we use the HAVING
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clause:
SELECT BUYERID, MAX(PRICE)
FROM ANTIQUES
GROUP BY BUYERID
HAVING PRICE > 1000; .
Example using the SUM function
For example, you could also use the SUM function to return the name of the department and the total sales (in the
associated department). The HAVING clause will filter the results so that only departments with sales greater than
$1000 will be returned.
SELECT department, SUM(sales) as "Total sales"
FROM order_details
GROUP BY department
HAVING SUM(sales) > 1000;
Example using the COUNT function
For example, you could use the COUNT function to return the name of the department and the number of employees
(in the associated department) that make over $25,000 / year. The HAVING clause will filter the results so that only
departments with more than 10 employees will be returned.
SELECT department, COUNT(*) as "Number of employees"
FROM employees
WHERE salary > 25000
GROUP BY department
HAVING COUNT(*) > 10;
Example using the MIN function
For example, you could also use the MIN function to return the name of each department and the minimum salary in
the department. The HAVING clause will return only those departments where the starting salary is $35,000.
SELECT department, MIN(salary) as "Lowest salary"
FROM employees
GROUP BY department
HAVING MIN(salary) = 35000;
Example using the MAX function
For example, you could also use the MAX function to return the name of each department and the maximum salary in
the department. The HAVING clause will return only those departments whose maximum salary is less than $50,000.
SELECT department, MAX(salary) as "Highest salary"
FROM employees
GROUP BY department
HAVING MAX(salary) < 50000;
More Subqueries
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Another common usage of subqueries involveswww.thecodexpert.com
the use of operators to allow a Where condition to include the Select
output of a subquery. First, list the buyers who purchased an expensive item (the Price of the item is $100 greater than
the average price of all items purchased):
SELECT BUYERID
FROM ANTIQUES
WHERE PRICE >
(SELECT AVG(PRICE) + 100
FROM ANTIQUES);
The subquery calculates the average Price, plus $100, and using that figure, an OwnerID is printed for every item
costing over that figure. One could use DISTINCT BUYERID, to eliminate duplicates.
List the Last Names of those in the AntiqueOwners table, ONLY if they have bought an item:
SELECT OWNERLASTNAME
FROM ANTIQUEOWNERS
WHERE OWNERID IN
(SELECT DISTINCT BUYERID
FROM ANTIQUES);
The subquery returns a list of buyers, and the Last Name is printed for an Antique Owner if and only if the Owner's ID
appears in the subquery list (sometimes called a candidate list). Note: on some DBMS's, equals can be used instead of
IN, but for clarity's sake, since a set is returned from the subquery, IN is the better choice.
For an Update example, we know that the gentleman who bought the bookcase has the wrong First Name in the
database...it should be John:
UPDATE ANTIQUEOWNERS
SET OWNERFIRSTNAME = 'John'
WHERE OWNERID =
(SELECT BUYERID
FROM ANTIQUES
WHERE ITEM = 'Bookcase');
First, the subquery finds the BuyerID for the person(s) who bought the Bookcase, then the outer query updates his First
Name.
Remember this rule about subqueries: when you have a subquery as part of a WHERE condition, the Select clause in
the subquery must have columns that match in number and type to those in the Where clause of the outer query. In
other words, if you have "WHERE ColumnName = (SELECT...);", the Select must have only one column in it, to match
the ColumnName in the outer Where clause, and they must match in type (both being integers, both being character
strings, etc.).
EXISTS & ALL
EXISTS uses a subquery as a condition, where the condition is True if the subquery returns any rows, and False if the
subquery does not return any rows; this is a nonintuitive feature with few unique uses. However, if a prospective
customer wanted to see the list of Owners only if the shop dealt in Chairs, try:
SELECT OWNERFIRSTNAME, OWNERLASTNAME
FROM ANTIQUEOWNERS
WHERE EXISTS
(SELECT *
FROM ANTIQUES
WHERE ITEM = 'Chair');
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If there are any Chairs in the Antiques column,www.thecodexpert.com
the subquery would return a row or rows, making the EXISTS clause
true, causing SQL to list the Antique Owners. If there had been no Chairs, no rows would have been returned by the
outside query.
ALL is another unusual feature, as ALL queries can usually be done with different, and possibly simpler methods; let's
take a look at an example query:
SELECT BUYERID, ITEM
FROM ANTIQUES
WHERE PRICE >= ALL
(SELECT PRICE
FROM ANTIQUES);
This will return the largest priced item (or more than one item if there is a tie), and its buyer. The subquery returns a list
of all Prices in the Antiques table, and the outer query goes through each row of the Antiques table, and if its Price is
greater than or equal to every (or ALL) Prices in the list, it is listed, giving the highest priced Item. The reason "=" must
be used is that the highest priced item will be equal to the highest price on the list, because this Item is in the Price list.
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