将两个mysql表合并为一行

Question

我正在尝试将几个表格组合成一行。

团队表：

+----+-------+
| id | team  |
+----+-------+
| 10 | Team1 |
| 11 | Team2 |
| 12 | Team3 |
+----+-------+

位置表：

+----+-----------+
| id | location  |
+----+-----------+
|  1 | location1 |
|  2 | location2 |
|  3 | location3 |
+----+-----------+

停止表：

+----+---------+-------------+---------------------+
| id | team_id | location_id |      timestamp      |
+----+---------+-------------+---------------------+
|  1 |      10 |           2 | 2019-11-07 15:27:42 |
|  2 |      10 |           3 | 2019-11-07 16:37:52 |
|  3 |      10 |           4 | 2019-11-07 17:47:62 |
+----+---------+-------------+---------------------+

希望创建所需的表：

+----+---------+---------------------+---------------------+---------------------+
| id | team_id |     (loc id=2)      |     (loc id=3)      |     (loc id=4)      |
+----+---------+---------------------+---------------------+---------------------+
|  1 |      10 | 2019-11-07 15:27:42 | 2019-11-07 16:37:52 | 2019-11-07 17:47:62 |
|  2 |      11 |                     |                     |                     |
|  3 |      12 |                     |                     |                     |
+----+---------+---------------------+---------------------+---------------------+

位置的数量总是有限的。

任何指导将不胜感激！我已经尝试了一些 LEFT JOINS，但还没有走远。

Answer 1

你可以做条件聚合：

select
    t.id team_id
    max(case when s.location_id = 2 then timestamp end) loc_id_2,
    max(case when s.location_id = 3 then timestamp end) loc_id_3,
    max(case when s.location_id = 4 then timestamp end) loc_id_4
from 
    team t
    left join stops s on s.team_id = t.id
group by t.id

如果您想为生成的结果动态生成一个id 列（这没什么意义，因为您已经为每个team_id 获得了一条记录），那么您可以使用row_number()（在MySQL 8.0 及更高版本中可用):

select 
    row_number() over(order by t.id) id,
    t.*
from (  
    select
        t.id team_id,
        max(case when s.location_id = 2 then timestamp end) loc_id_2,
        max(case when s.location_id = 3 then timestamp end) loc_id_3,
        max(case when s.location_id = 4 then timestamp end) loc_id_4
    from 
        team t
        left join stops s on s.team_id = t.id
    group by t.id
) t