| Lesson 3 || SQL WHERE clause |
| Objective || Use WHERE to select specific rows of a table. |
SQL WHERE Clause
Often times you will want queries to return only a subset of the entire database. For example, when searching for a list of books by a specific author.
The SELECT statement's WHERE clause allows you to select only data from rows that satisfy a certain condition.
A clause further defines a statement. For example the following statement returns all rows:
SELECT * FROM BookTable
Adding the WHERE clause such as WHERE Author='Hemingway', further defines the statement to return only rows in which the author equals Hemingway. The following SQL query returns books by Hemingway.
SELECT * FROM BookTable WHERE Author = 'Hemingway'
The WHERE clause uses comparison and logical operators to select rows meeting the specified condition. The following SlideShow describes how the WHERE clause is used:
Using Where Clause
SQL has several operators that allow the SELECT statement to be very selective
about the data returned from a query.
The following tables list the comparison and logical operators used with SQL:
| Comparison Operators |
| Symbol || Function |
|=|| Equal |
| <> || Not equal |
| > || Greater than |
| >=|| Greater than or equal |
| < || Less than |
| <=|| Less than or equal |
| Logical Operators |
| Operator || Function |
| AND || Returns TRUE if all conditions are true |
| OR || Returns TRUE if one or more conditions are true |
| NOT || Returns TRUE if the condition is false |
The following statement uses the logical operator AND to select rows where state equals Arizona and last name equals Henry:
SELECT * FROM CustTable WHERE ST = 'AZ' AND
LName = 'Henry'
In the next lesson, the ORDER BY clause to sort rows in a table will be discussed.