> -----Original Message-----
> From: Dan Nelson [mailto:dnelson@stripped]
> Sent: 15 June 2007 19:24
> To: Edward Kay
> Cc: MySQL List
> Subject: Re: Hiding columns used in GROUP BY and HAVING clauses
>
>
> In the last episode (Jun 15), Edward Kay said:
> > I have a table of addresses. Each address is associated with a primary
> > entity and a primary entity can have n different addresses. For
> each primary
> > entity, one address is marked as the main address.
> >
> > I need a query to return all addresses that are the only
> address associated
> > with the primary entity but aren't marked as the main address.
> >
> > At the moment, I have this and it works:
> >
> > select * from contact_address
> > group by primary_entity_id
> > having count(primary_entity_id) = 1
> > and is_primary = 0;
> >
> > This is fine except I want to use the result in a sub-query. Since it
> > returns two columns this doesn't work:
> >
> > update contact_address set is_primary = 1 where address_id in (
> > select * from contact_address
> > group by primary_entity_id
> > having count(primary_entity_id) = 1
> > and is_primary = 0
> > );
> >
> > Normally, I'd only return the address_id in the sub-SELECT, but I
> > need the is_primary column for the HAVING clause.
>
> I did some tests, and it looks like you can use aggregate functions in
> your HAVING clause without actually selecting the column. So "HAVING
> COUNT(primary_entity_id) = 1" should work even if you only select
> address_id.
>
Yes, that is true and it does work.
What doesn't work however, is the extra 'AND is_primary = 0' HAVING clause.
Thanks anyway,
Edward
