Mysql 如何在更少的查询中高效地为多个列多次连接同一个表？

我必须在mysql查询中多次连接一个表，我不想对每一列进行左连接。我的表和sql是-
表(mst)-

储物架-

SQL-

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如何在不使用store\u pick table的多个左联接的情况下执行此操作？

不需要所有这些联接，您可以使用如下表达式：

SELECT
  tm.name,
  MAX(CASE WHEN sp1.pick_id = tm.result1 THEN sp1.pick_name END) AS Result1,
  MAX(CASE WHEN sp1.pick_id = tm.result2 THEN sp1.pick_name END) AS Result2,
  MAX(CASE WHEN sp1.pick_id = tm.result3 THEN sp1.pick_name END) AS Result3
FROM Table_mst as tm
LEFT JOIN store_pick AS sp1  ON sp1.store_id = 'result'
GROUP BY tm.name;

您将得到与原始查询完全相同的结果：

| name | Result1 | Result2 | Result3 |
|------|---------|---------|---------|
|    A |    pass |  supply |  supply |
|    B |    fail |    fail |    fail |
|    C |   grace |   grace |    pass |
|    D |     ufm |  supply |    fail |
|    E |    fail |    fail |    fail |

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规范化数据。预期结果是什么？现有查询有什么问题？是的，你好。做什么？我们无法读懂你的心思。请阅读并采取行动。这也是一个常见问题，谷歌我的评论再谷歌问题。像这样使用

CASE

对小表很有效，这是解决这个问题的好方法。我只能补充一点，你可以把它放在一个视图中，以重用代码给读者：这个答案被称为条件聚合，这是一种常用的数据透视（从长到宽）查询数据的方法。

 SELECT
  tm.name,
  sp1.pick_name AS result1,
  sp2.pick_name AS result2,
  sp3.pick_name AS result3,
  sp4.pick_name AS result4
FROM table_mst AS tm
  LEFT JOIN store_pick AS sp1
    ON sp1.pick_id = tm.result1
      AND sp1.store_id = 'result'
  LEFT JOIN store_pick AS sp2
    ON sp2.pick_id = tm.result2
      AND sp2.store_id = 'result'
  LEFT JOIN store_pick AS sp3
    ON sp3.pick_id = tm.result3
      AND sp3.store_id = 'result'
  LEFT JOIN store_pick AS sp4
    ON sp4.pick_id = tm.result4
      AND sp4.store_id = 'result'

SELECT
  tm.name,
  MAX(CASE WHEN sp1.pick_id = tm.result1 THEN sp1.pick_name END) AS Result1,
  MAX(CASE WHEN sp1.pick_id = tm.result2 THEN sp1.pick_name END) AS Result2,
  MAX(CASE WHEN sp1.pick_id = tm.result3 THEN sp1.pick_name END) AS Result3
FROM Table_mst as tm
LEFT JOIN store_pick AS sp1  ON sp1.store_id = 'result'
GROUP BY tm.name;

| name | Result1 | Result2 | Result3 |
|------|---------|---------|---------|
|    A |    pass |  supply |  supply |
|    B |    fail |    fail |    fail |
|    C |   grace |   grace |    pass |
|    D |     ufm |  supply |    fail |
|    E |    fail |    fail |    fail |