Peter Brawley wrote:
> >I want to find all A's such that
> >they have exactly B's 1 and 2
> >SELECT A.ID, group_concat(BID ORDER BY BID) as Bs
> >FROM A INNER JOIN AhasB ON A.ID=AID
> >GROUP BY A.ID
> >HAVING Bs='1,2'
> Why the join? Doesn't your ahasb bridge table already incorporate the
> join logic? If your requirement is to retrieve all aid's with exactly
> one instance of bid=1, exactly one with bid=2, and no other bid's, why
> not just ...
> SELECT aid,GROUP_CONCAT(bid) AS list
> FROM ahasb
> GROUP BY aid
> HAVING list='1,2';
I actually need some of the other columns from A, but you're correct,
this does work. I did discover though that the ORDER BY in the
group_concat is important, since MySQL doesn't always pick the same
order for the list.
> I think this will work:
> SELECT AID
> FROM AhasB
> WHERE BID in (1,2)
> GROUP BY AID
> HAVING count(BID) =2
Thank you, this is actually very helpful. The where clause uses the
index I have in the table to screen out many more rows early in the
query, and it also very nicely avoids the string compare on the
group_concat. I also wasn't aware that you could use an aggregate
function in the HAVING clause without it appearing in the SELECT clause.
Thanks again, this solves the problem quite elegantly, and I probably
never would have thought of it.