SQL查询以查找两列中不同值的计数

Question

给出下面名为demo的表：

+----+-------+--------+
| id | bag_1 | bag_2  |
+----+-------+--------+
|  1 | Alpha | Alpha  |
|  2 | Alpha | Beta   |
|  3 | Beta  | Beta   |
|  4 | Beta  | Omega  |
|  5 | Beta  | Delta  |
|  6 | Beta  | Sigma  |
|  7 | Gamma | Alpha  |
|  8 | Gamma | Lambda |
|  9 | Gamma | Omega  |
+----+-------+--------+

如何在bag_1和bag_2列中找到不同值的计数？

（预期计数= 7）

注意：原始表有大约180万行，因此连接从未成功完成。

Answer 1

可以使用UNION ALL收集所有可能的值，然后COUNT(DISTINCT bag)将完成其余的操作，如下所示：

SELECT COUNT(DISTINCT bag) from (SELECT bag_1 AS bag FROM demo UNION ALL SELECT bag_2 AS bag FROM demo) a;

+---------------------+ | count(distinct bag) | +---------------------+ | 7 | +---------------------+

编辑： @Tim Biegeleisen和@eggyal的屈膝礼，一个较短的形式是：

SELECT COUNT(*) from (SELECT bag_1 AS bag FROM demo UNION SELECT bag_2 AS bag FROM demo) a;