|SQL Language Reference / SQL Clauses|
Select rows based on a boolean expression.
WHERE Boolean expressions
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.
-- 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