SQL查詢以查找兩列中不同值的計數

Question

給定名為demo的下表：

+----+-------+--------+
| 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列中找到不同值的計數？

（預期數量= 7）

注意：原始表有約180萬行，因此聯接從未成功完成。

Answer 1

可以使用UNION ALL收集所有可能的值，然后COUNT(DISTINCT bag)將完成其余的工作，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 | +---------------------+

編輯：@Tim Biegeleisen和@eggyal的禮貌，較短的形式是：

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