mysql - 意外结果 - MySQL SELECT 基于日期比较

Question

我在使用 MySQL 选择查询时遇到了一些意外行为。我正在运行查询:

SELECT `refno`, `subdomain`, `toplevels`, `renew_until`, `expiry_date`, 
(YEAR(`renew_until`) - YEAR(`expiry_date`)) AS `renew_for` FROM `testing_names` 
WHERE `expiry_date` >= DATE(NOW()) AND `renew_for` >= 0

返回(如预期):

|  refno  |  subdomain  |  toplevels  |  renew_until  |  expiry_date  |  renew_for  |
|-----------------------------------------------------------------------------------|
|  5      |  domain1    |  com        |  2014-02-02   |  2014-02-02   |  0          |
|  45     |  domain2    |  net        |  2014-01-27   |  2013-01-27   |  1          |

但是下面的查询(注意 renew_for 的不同比较)返回一个空集:

SELECT `refno`, `subdomain`, `toplevels`, `renew_until`, `expiry_date`, 
(YEAR(`renew_until`) - YEAR(`expiry_date`)) AS `renew_for` FROM `testing_names` 
WHERE `expiry_date` >= DATE(NOW()) AND `renew_for` > 0

在这种情况下，我期待第 45 行；我的查询有什么问题？我是否以正确的方式使用 renew_for？

Answer 1

其他 RDBMS 不允许此语法:列别名不应在 WHERE 子句中可用。
MySQL does it's own thing though结果不稳定:

Standard SQL doesn't allow you to refer to a column alias in a WHERE clause. This restriction is imposed because when the WHERE code is executed, the column value may not yet be determined.

所以，把WHERE条件改成这样

WHERE `expiry_date` >= DATE(NOW()) AND (YEAR(`renew_until`) - YEAR(`expiry_date`)) > 0

或者使用派生表

SELECT *
FROM
   (
    SELECT 
        `refno`, `subdomain`, `toplevels`, `renew_until`, `expiry_date`, 
         (YEAR(`renew_until`) - YEAR(`expiry_date`)) AS `renew_for`
    FROM `testing_names` 
    ) T
WHERE `expiry_date` >= DATE(NOW()) AND `renew_for` > 0