如何在更少的查询中有效地为多个列多次连接同一表？

Question

我必须在mysql查询中多次联接一个表，但我不想为每一列做左联接。 我的表和SQL是-
table_mst-

id  name    result1 result2 result3
1    A       1       3       3
2    B       2       2       2
3    C       4       4       1
4    D       5       3       2
5    E       2       2       2

store_pick-

id  store_id    pick_id pick_name
1   result       1      pass
2   result       2      fail
3   result       3      supply
4   result       4      grace
5   result       5      ufm

SQL-

 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'

如何不对store_pick表使用多个左联接？

Answer 1

不需要所有这些连接，您可以使用CASE表达式，如下所示：

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;

• 样本演示

您将获得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 |