Using the EXCEPT keyword

Suppose that you want to find a list of employee numbers that includes people in department D11 minus those people whose assignments include projects MA2112, MA2113, and AD3111.

This query returns all of the people in department D11 who are not working on projects MA2112, MA2113, and AD3111:

SELECT EMPNO
   FROM CORPDATA.EMPLOYEE
   WHERE WORKDEPT = 'D11'
 EXCEPT
 SELECT EMPNO
   FROM CORPDATA.EMPPROJACT
   WHERE PROJNO = 'MA2112' OR
      PROJNO = 'MA2113' OR
      PROJNO = 'AD3111'
 ORDER BY EMPNO

To better understand the results from these SQL statements, imagine that SQL goes through the following process:

Step 1. SQL processes the first SELECT statement:

SELECT EMPNO
     FROM CORPDATA.EMPLOYEE
     WHERE WORKDEPT = 'D11'

This query returns an interim result table.

Step 2. SQL processes the second SELECT statement:

SELECT EMPNO
     FROM CORPDATA.EMPPROJACT
     WHERE PROJNO='MA2112' OR
                 PROJNO= 'MA2113' OR
                 PROJNO= 'AD3111'

This query returns another interim result table.

Step 3. SQL takes the first interim result table, removes all of the rows that also appear in the second interim result table, removes duplicate rows, and orders the result:

SELECT EMPNO 
     FROM CORPDATA.EMPLOYEE
     WHERE WORKDEPT = 'D11'
EXCEPT
SELECT EMPNO 
     FROM CORPDATA.EMPPROJACT
     WHERE PROJNO='MA2112' OR
                 PROJNO= 'MA2113' OR
                 PROJNO= 'AD3111'
ORDER BY EMPNO

This query returns a combined result table with values in ascending sequence.