Edward Kay wrote:
-----Original Message-----
From: Dan Nelson [mailto:[EMAIL PROTECTED]
Sent: 18 June 2007 15:11
<snip>
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.
Couldn't you move that up into a WHERE clause (still in the subquery)?
It's just a regular field comparison, so it doesn't have to be in the
HAVING clause.
No, because that's not the same thing. Imagine the following data:
address_id primary_entity_id is_primary
-------------------------------------------
1 293 0
2 293 1
With my query above, the sub-query result set would be empty since
count(primary_entity_id) = 2.
If I move the is_primary = 0 requirement to a WHERE clause, then the first
row would be incorrectly updated since the group function would only be
working on the sub-set of data (where is_primary = 0).
It's a subtle but important difference.
Then I think what you really want in your HAVING clause is "AND MIN(is_primary)
= 0".
Baron
--
MySQL General Mailing List
For list archives: http://lists.mysql.com/mysql
To unsubscribe: http://lists.mysql.com/[EMAIL PROTECTED]
