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Filtering on an alias in mysql

开发者 https://www.devze.com 2023-01-26 23:38 出处:网络
Why doesn\'t the following query work? Mysql complains about z - can\'t I use an alias in the WHERE clause?

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 e开发者_运维问答rror that I get is:

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


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;


You should be aware that the usage of the HAVING clause without a GROUP BY clause is a non-standard extension of MySQL and won't work in other databases.

If you want this to be portable, you need to use a derived table:

SELECT *
FROM (
  SELECT (x + y) AS z, t.*  
  FROM t
  WHERE x = 1 
) t2
WHERE z = 2


Use having clause:

SELECT (x + y) AS z, t.*  FROM t
WHERE 
x = 1 
having z=2
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