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 = 2

The error that I get is:

Error Code : 1054
Unknown column 'z' in 'where clause'

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;