过滤mysql中的别名

Question

为什么以下查询无效？Mysql抱怨z - 我不能在WHERE子句中使用别名吗？

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

我得到的错误是:

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

Answer 1

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

标准SQL不允许在WHERE子句中引用列别名.强制执行此限制是因为在评估WHERE子句时,可能尚未确定列值.例如,以下查询是非法的:

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

试试这个,相反:

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