Filtering on an alias in mysql

Question

Why doesn\'t the following query work? Mysql complains about z - can\'t I use an alias in the WHERE clause?

SELECT x + y AS z, t.*  FROM t
WHERE 
x = 1 and
z         


        

Answer 1

http://dev.mysql.com/doc/refman/5.0/en/problems-with-alias.html

Standard SQL disallows references to column aliases in a WHERE clause. This restriction is imposed because when the WHERE clause is evaluated, the column value may not yet have been determined. For example, the following query is illegal:

SELECT id, COUNT(*) AS cnt FROM tbl_name WHERE cnt > 0 GROUP BY id;

Try this, instead:

SELECT x + y AS z, t.* FROM t WHERE x = 1 HAVING z = 2;