MySQL DELETE FROM with UNION subquery by IN condit

问题:

I have tripped up on a curious SQL error. The last query doesn't work. Of course I can just split that DELETE into three queries, but I really wonder why MySQL doesn't let me do it this way.

A little example:

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

works!

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

works, too!

Whereas

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)
)

raises the error

#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

Anyone a clue?

回答1:

Try that SQL

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)

回答2:

Try this version instead:

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)
                )

I think the issue is an arcane issue with the definition of a subquery. A subquery is a select statement, whereas a union is a conjunction of select statements.

EDIT:

Actually, if you want efficiency, you wouldn't use this approach at all. I was just trying to show how to fix the error. A better solution would be:

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);

Indexes are recommended on stairs(id, building), lift(id, building), and qrcodeid(id, building).