# WHERE clause

A WHERE clause is an optional part of a SelectExpression, DELETE statement, or UPDATE statement. The WHERE clause lets you select rows based on a boolean expression. Only rows for which the expression evaluates to TRUE are returned in the result, or, in the case of a DELETE statement, deleted, or, in the case of an UPDATE statement, updated.

#### Syntax

    WHERE Boolean expression

Boolean expressions are allowed in the WHERE clause. Most of the general expressions listed in Table of general expressions, can result in a boolean value.

In addition, there are the more common boolean expressions. Specific boolean operators listed in Table 10, take one or more operands; the expressions return a boolean value.

#### Example

    -- find the flights where no business-class seats have
    -- been booked
    SELECT *
    FROM FlightAvailability
    WHERE business_seats_taken IS NULL
    OR business_seats_taken = 0
    -- Join the EMP_ACT and EMPLOYEE tables
    -- select all the columns from the EMP_ACT table and
    -- add the employee's surname (LASTNAME) from the EMPLOYEE table
    -- to each row of the result.
    SELECT SAMP.EMP_ACT.*, LASTNAME
      FROM SAMP.EMP_ACT, SAMP.EMPLOYEE
      WHERE EMP_ACT.EMPNO = EMPLOYEE.EMPNO
    -- Determine the employee number and salary of sales representatives
    -- along with the average salary and head count of their departments.
    -- This query must first create a new-column-name specified in the AS clause
    -- which is outside the fullselect (DINFO)
    -- in order to get the AVGSALARY and EMPCOUNT columns,
    -- as well as the DEPTNO column that is used in the WHERE clause
    SELECT THIS_EMP.EMPNO, THIS_EMP.SALARY, DINFO.AVGSALARY, DINFO.EMPCOUNT
     FROM EMPLOYEE THIS_EMP,
       (SELECT OTHERS.WORKDEPT AS DEPTNO,
               AVG(OTHERS.SALARY) AS AVGSALARY,
               COUNT(*) AS EMPCOUNT
        FROM EMPLOYEE OTHERS
        GROUP BY OTHERS.WORKDEPT
       )AS DINFO
     WHERE THIS_EMP.JOB = 'SALESREP'
    	   AND THIS_EMP.WORKDEPT = DINFO.DEPTNO

Parent topic: SQL clauses