Question

我已经完成了一些工作，并且经常看到这样的数据库：

            Column_Values
            +------+--------------+--------------+
            |  id  |   field_id   |    label     |
            +------------------------------------+
            |  1   |  first_name  |  First Name  +-----------------+
            |  2   |  last_name   |  Last Name   +-----------------------+
            |      |              |              |                 |     |
            |      |              |              |                 |     |
            |      |              |              |                 |     |
            +------+--------------+--------------+                 |     |
                                                                   |     |
            User Row                                               |     |
            +-------+                                              |     |
            |  id   |                                              |     |
            +-------+                                              |     |
+-----------+ 9000  |                                              |     |
|           | 10000 |                                              |     |
|           |       |                                              |     |
|           |       |                                              |     |
|           |       |                                              |     |
|           +-------+                                              |     |
|                                                                  |     |
|           Mapping Table                                          |     |
|           +------------+-------------+---------+                 |     |
|           |  User_Id   |  Field ID   |  Value  |                 |     |
|           +------------------------------------+                 |     |
+----------->    9000    |     1       |  John   <-----------------+     |
            |    9000    |     2       |   Doe   <-----------------------+
            +------------+-------------+---------+

该列的名称实际上存储在另一个表中，而不是实际有一个first_name和一个last_name的列。

如何选择用户，以便获得类似的信息？

+---------+-------------+-----------+
| User_ID | First Name  | Last Name |
+-----------------------------------+
| 9000    |  John       |   Doe     |
|         |             |           |
+---------+-------------+-----------+

还有这种数据库的名称吗？似乎背后的逻辑是拥有动态列，但是我认为可以通过计划内的停机时间来更新表来缓解此问题。

Answer 1

条件聚合应该可以解决问题：

SELECT
    u.id,
    MAX(CASE WHEN cv.field_id = 'first_name' THEN m.value END) first_name,
    MAX(CASE WHEN cv.field_id = 'last_name' THEN m.value END) last_name
FROM user u
INNER JOIN mapping m ON m.user_id = u.id 
INNER JOIN Column_Values cv ON cv.id = m.field_id
GROUP BY u.id

注意：如果您只需要用户ID及其名字和姓氏，那么就不需要输入user表。只是：

SELECT
    m.user_id,
    MAX(CASE WHEN cv.field_id = 'first_name' THEN m.value END) first_name,
    MAX(CASE WHEN cv.field_id = 'last_name' THEN m.value END) last_name
FROM mapping m ON 
INNER JOIN Column_Values cv ON cv.id = m.field_id
GROUP BY m.user_id

Answer 2

尝试一下

select 
  User_ID, 
  max(FirstName) FirstName, 
  max(LastName) LastName
from 
(
  select 
    User_ID,
    case when c.Field_Id= 'first_name' then Value end as FirstName,
    case when c.Field_Id = 'last_name' then Value end as LastName    
  from Column_Values c
  inner join MappingTable m
    on c.Id = m.FieldId
) t
group by
  User_ID

SQL-选择值成为列值？

2 个答案: