From: Dan Nelson
Date: June 15 2007 6:24pm
Subject: Re: Hiding columns used in GROUP BY and HAVING clauses
List-Archive: http://lists.mysql.com/mysql/207514
Message-Id: <20070615182418.GD30806@dan.emsphone.com>
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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.  

-- 
	Dan Nelson
	dnelson@stripped