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[英]MySQL Selecting multiple rows with the same joining ID from the same table
[英]Joining multiple rows with same ID in one
我在使用 SQL 查询时遇到问题。 我有两张桌子。
我的第一张桌子:
+------------+-------------+---------------+
| id_mission | Some column | Other column |
+------------+-------------+---------------+
| 1 | ... | ... |
| 2 | ... | ... |
+------------+-------------+---------------+
我的第二张桌子:
+------------+-------------+---------+
| id_mission | id_category | points |
+------------+-------------+---------+
| 1 | 1 | 3 |
| 1 | 2 | 4 |
| 1 | 3 | 4 |
| 1 | 4 | 8 |
| 2 | 1 | -4 |
| 2 | 2 | 3 |
| 2 | 3 | 1 |
| 2 | 4 | -7 |
+------------+-------------+---------+
我想在我的SELECT
请求中得到这种结果
+------------+-------------+--------------+---------------+----------------+
| id_mission | Some column | Other column | id_category 1 | id_category X |
+------------+-------------+--------------+---------------+----------------+
| 1 | ... | ... | ... | ... |
| 2 | ... | ... | ... | ... |
+------------+-------------+--------------+---------------+----------------+
我在前两列中尝试过这个,但它不起作用,我也尝试过GROUP_CONCAT
,它有效,但不是我想要的结果。
SELECT m.id_mission ,mc.id_category 1,mc1.id_category 2
from mission m
left join mission_category mc on m.id_mission = mc.id_mission
left join mission_category mc1 on m.id_mission = mc1.id_mission
有人能帮我吗?
您可以使用条件聚合。 假设您要对每个类别的points
值进行透视:
select
t1.*,
max(case when t2.id_category = 1 then points end) category_1,
max(case when t2.id_category = 2 then points end) category_2,
max(case when t2.id_category = 3 then points end) category_3
from t1
inner join t2 on t2.id_mission = t1.id_mission
group by t1.id_mission
这假设id_mission
是t1
的主键(否则,您需要在select
和group by
子句中枚举所需的列)。
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