MySQL DELETE FROM with UNION 子查询按 IN 条件

Question

我遇到了一个奇怪的 SQL 错误。最后一个查询不起作用。当然，我可以将 DELETE 拆分为三个查询，但我真的很奇怪为什么 MySQL 不允许我这样做。

一个小例子:

(SELECT id FROM stairs WHERE building = 123)
UNION
(SELECT id FROM lift WHERE building = 123)
UNION
(SELECT id FROM qrcodeid WHERE building = 123)

有效!

DELETE FROM startpoint WHERE id IN (SELECT id FROM stairs WHERE building = 123)

同样有效!

鉴于

DELETE FROM startpoint WHERE id IN (
    (SELECT id FROM stairs WHERE building = 123)
    UNION
    (SELECT id FROM lift WHERE building = 123)
    UNION
    (SELECT id FROM qrcodeid WHERE building = 123)
)

引发错误

#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'UNION (SELECT id FROM lift WHERE building = 123) UNION (SELECT id FROM qrc' at line 3

有人知道吗？

Answer 1

试试这个版本:

DELETE FROM startpoint
    WHERE id IN (select *
                 from ((SELECT id FROM stairs WHERE building = 123)
                       UNION
                      (SELECT id FROM lift WHERE building = 123)
                       UNION
                      (SELECT id FROM qrcodeid WHERE building = 123)
                )

我认为这个问题是关于子查询定义的神秘问题。子查询是 select 语句，而 union 是 select 语句的结合。

编辑:

实际上，如果你想要效率，你根本不会使用这种方法。我只是想展示如何修复错误。更好的解决方案是:

DELETE sp FROM startpoint sp
    WHERE EXISTS (select 1 from stairs s where s.building = 123 and s.id = sp.id) or
          EXISTS (select 1 from lift l where l.building = 123 and l.id = sp.id) or
          EXISTS (select 1 from qrcodeid q where q.building = 123 and q.id = sp.id);

建议在 stairs(id, building)、lift(id, building) 和 qrcodeid(id, building) 上使用索引。