Question

I’m having the following tables:

Table a

Field	Type	Null	Key
bid	int(10) unsigned	YES
cid	int(10) unsigned	YES

Table b

Field	Type	Null
bid	int(10) unsigned	NO
cid	int(10) unsigned	NO
data	int(10) unsigned	NO

When I want to select all rows from b where there’s a corresponding bid/cid-pair in a, I simply use a natural join SELECT b.* FROM b NATURAL JOIN a; and everything is fine.

When a.bid or a.cid is NULL, I want to get every row where the other column matches, e.g. if a.bid is NULL, I want every row where a.cid = b.cid, if both are NULL I want every column from b.

My naive solution was this:

SELECT DISTINCT b.*
FROM b
JOIN a ON (ISNULL(a.bid) OR a.bid=b.bid ) AND (ISNULL(a.cid) OR a.cid=b.cid)

Is there any better way to to this?

Answer 1

No, that’s pretty much it.

(I’d generally rephrase ISNULL(a.bind) as a.bind IS NULL for ANSI SQL compliance FWIW.)