mysql - 在此查询中，如何在 GROUP BY 之前执行 SELECT？

Question

我在看SQL的执行顺序，发现是:

FROM,
WHERE,
GROUP BY,
HAVING,
SELECT,
ORDER BY

但在下面的查询中，“_index”用于 GROUP BY，这怎么可能？

SELECT COUNT(ab.id) AS count, COUNT(ab.id)/365.24 AS average,
    IF((SUBSTR(ab.begin, 1, 7) = '2014-08'), '2014-08-18 00:00:00.0 CEST',
    IF((SUBSTR(ab.begin, 1, 7) = '2014-09'), '2014-09-18 00:00:00.0 CEST',
    IF((SUBSTR(ab.begin, 1, 7) = '2014-10'), '2014-10-18 00:00:00.0 CEST',
    IF((SUBSTR(ab.begin, 1, 7) = '2014-11'), '2014-11-18 00:00:00.0 CET',
       '0')))) AS _index
FROM active_begin AS ab 
INNER JOIN asources AS a ON a.id = ab.asource AND a.unit IN (4, 3, 1) 
WHERE (1408226400000 <= ab.begin_time AND ab.begin_time < 1417388400000)
GROUP BY _index

附言。引用此订单:http://www.bennadel.com/blog/70-sql-query-order-of-operations.htm提前致谢。

Answer 1

我认为这(在 GROUP BY 的 SELECT 子句中定义的 ability to use a column alias)可能是某些数据库(但不是全部)允许的非标准扩展。

您应该再次重复确切的定义或将所有内容包装在子选择中。

如果您的数据库能让您摆脱困境，那就太幸运了。