Slow MySQL queries?

[Azu]

Would somebody please tell me why these MySQL queries are running so slow and how I can optimize them?

 

select count(*)

from Z where X not in (select 1 from a) and X not in (select 2 from b) and X not in (select 3 from c);

 

delete

from Z where X not in (select 1 from a) and X not in (select 2 from b) and X not in (select 3 from c);

[bubblegum.anarchy]

Possibly?:

SELECT count(*) FROM Z
WHERE X NOT IN (
    SELECT 1 FROM a
    UNION
    SELECT 2 FROM b
    UNION
    SELECT 3 FROM c
)

 

EDIT: the above query is not faster.

[bubblegum.anarchy]

This is pretty much the same as your original query Azu:

 

SELECT count(*) FROM Z
    LEFT JOIN a ON Z.1 = a.1
    LEFT JOIN b ON Z.1 = b.1
    LEFT JOIN c ON Z.1 = c.1
WHERE a.1 IS NULL AND b.1 IS NULL AND c.1 IS NULL