When using the SELECT statement, you may not want duplicate information to be returned. For example, STAFF has a DEPT column in which several department numbers are listed more than once, and a JOB column in which several job descriptions are listed more than once.
To eliminate duplicate rows, use the DISTINCT option on the SELECT clause. For example, if you insert DISTINCT into the statement, each job within a department is listed only once:
SELECT DISTINCT DEPT, JOB
FROM STAFF
WHERE DEPT < 30
ORDER BY DEPT, JOB
This statement produces the following result:
DEPT JOB
------ -----
10 Mgr
15 Clerk
15 Mgr
15 Sales
20 Clerk
20 Mgr
20 Sales
DISTINCT has eliminated all rows that contain duplicate data in the set of columns specified in the SELECT statement.