小编Den*_*nis的帖子

无法理解GROUP BY子句的奇怪行为：

SELECT id, SUM(num) as sum
FROM (
    SELECT 1 AS id, 2 AS num UNION
    SELECT 1, 3) AS a
GROUP BY id

结果是：

SELECT id, SUM(num) as sum
FROM (
    SELECT 1 AS id, 2 AS num UNION
    SELECT 1, 3) AS a
GROUP BY id

但

SELECT id, SUM(num) as sum
FROM (
    SELECT 1 AS id, 2 AS num UNION
    SELECT 1, 2 /*!*/) AS a
GROUP BY id

结果是：

id  | sum …