The optional AS clause lets you assign a meaningful name to an expression, which makes referring back to the expression easier. You can use an AS clause to provide a name for any item in the select list.
The following statement displays all employees whose salary plus commission is less than $13, 000. The expression SALARY + COMM is named PAY:
SELECT NAME, JOB, SALARY + COMM AS PAY
FROM STAFF
WHERE (SALARY + COMM) < 13000
ORDER BY PAY
This statement produces the following result:
NAME JOB PAY
--------- ----- ----------
Yamaguchi Clerk 10581.50
Burke Clerk 11043.50
Scoutten Clerk 11592.80
Abrahams Clerk 12246.25
Kermisch Clerk 12368.60
Ngan Clerk 12714.80
By using the AS clause, you can refer to a particular column name rather than the system generated number in the ORDER BY clause. In this example we compare (SALARY + COMM) with 13000 in the WHERE clause, instead of using the name PAY. This is a result of the order of operations. The WHERE clause is evaluated before (SALARY + COMM) is given the name PAY. Hence, PAY cannot be used in the predicate.