使用联合选择构建 MySQL 视图

Posted 2023-02-26

【中文标题】使用联合选择构建 MySQL 视图

【英文标题】：Building a MySQL view with union of selects

【发布时间】：2017-05-15 12:07:02

【问题描述】：

我有一个具有以下结构的表： 列名称是指标（MT_NAME、MT_ADDRESS、MT_PHONE 等），行是来自该事件的指标的值/分数（0、6、8、9、10）。 像这样：

+---------+------------+----------+
| MT_NAME | MT_ADDRESS | MT_PHONE |
+---------+------------+----------+
|       0 |          9 |        6 |
|       6 |         10 |        0 |
|       8 |         10 |        0 |
+---------+------------+----------+ 

我想构建一个包含 3 列的视图：一列包含表列的名称，一列包含该列的不同值，另一列包含这些值在该列中出现的频率。 像这样的：

+------------+----------+-------+
|   LEGEND   | CATEGORY | VALUE |
+------------+----------+-------+
| MT_NAME    |     0    |     1 |
| MT_NAME    |     6    |     1 |
| MT_NAME    |     8    |     1 |
| MT_ADDRESS |     9    |     1 |
| MT_ADDRESS |     10   |     2 |
| MT_PHONE   |     0    |     2 |
| MT_PHONE   |     6    |     1 |
+------------+----------+-------+

有人可以帮忙吗？

【参考方案1】：

你可以使用union all:

select 'MT_NAME' as legend, mt_name as category, count(*) as frequency
from t
group by mt_name
union all
select 'MT_ADDRESS' as legend, mt_address as category, count(*) as frequency
from t
group by mt_name
union all
select 'MT_PHONE' as legend, mt_phone as category, count(*) as frequency
from t
group by mt_name;

【讨论】：

非常感谢！