为什么 GROUP BY 子句缺少行

Question

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 |
----|-----|
1   | 5   |

Answer 1

UNION 排除重复行。

你想UNION ALL在这个例子中使用：

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

结果：

id          sum
----------- -----------
1           4