MySQL GROUP BY 和 COUNT 多列

Question

我有一个 MySQL 表如下:

+----+-------+-------+-------+-------+-------+
| ID |  MON  |  TUE  |  WED  |  THU  |  FRI  |
+----+-------+-------+-------+-------+-------+
|  0 |  Bike |  Bike |  Walk |  Bike |  Car  |
|  1 |  Car  |  Car  |  Car  |  Bus  |  Car  |
|  2 |  Bus  | Train |  Bus  |  Bus  | Train |
|  3 |  Car  |  Car  |  Car  |  Walk |  Car  |
+----+-------+-------+-------+-------+-------+

我将如何分组并计算所有天数，以获得一周内每种交通工具的总模式。例如:

+--------+-------+
|  MODE  | COUNT |
+--------+-------+
|  Bike  |   3   |
|   Bus  |   4   |
|   Car  |   9   |
|  Train |   2   |
|  Walk  |   2   |
+--------+-------+

我试过使用:

SELECT COUNT(*), Mon 
FROM transport 
GROUP BY Mon, Tue, Wed, Thu, Fri

但这会为每天的每个唯一值创建一个新组。

Answer 1

一种方法是生成一个子查询，使用 union all 运算符在一列中选择传输模式，然后计算出现次数:

SELECT   mode, COUNT(*)
FROM     (SELECT mon AS mode FROM transport UNION ALL
          SELECT tue AS mode FROM transport UNION ALL
          SELECT wed AS mode FROM transport UNION ALL
          SELECT thu AS mode FROM transport UNION ALL
          SELECT fri AS mode FROM transport) t
GROUP BY mode