Re: subquery/alias question

Michael Glaesemann wrote:
>
> On Sep 25, 2007, at 17:30 , Alvaro Herrera wrote:
>
>> Michael Glaesemann wrote:
>>>
>>> select dom_id,
>>> dom_name,
>>> usr_count
>>> from domains
>>> natural join (select usr_dom_id as dom_id,
>>> count(usr_dom_id) as usr_count
>>> from users) u
>>> where usr_count > 0
>>> order by dom_name;
>>
>> Maybe the usr_count should be tested in a HAVING clause instead of
>> WHERE? And put the count(*) in the result list instead of a subselect.
>> That feels more natural to me anyway.
>
> I believe you'd have to write it like
>
> select dom_id, dom_name, count(usr_dom_id) as usr_count
> from domains
> join users on (usr_dom_id = dom_id)
> having count(usr_dom_id) > 0
> order by dom_name;
>
> I don't know how the performance would compare. I think the backend is
> smart enough to know it doesn't need to perform two seq scans to
> calculate count(usr_dom_id), but I wasn't sure.
>
> Madison, how do the two queries compare with explain analyze?

Thanks for your reply!

Unfortunately, in both cases I get the error:

nmc=> SELECT dom_id, dom_name, COUNT(usr_dom_id) AS usr_count FROM
domains JOIN users ON (usr_dom_id=dom_id) HAVING COUNT (usr_dom_id) > 0
ORDER BY dom_name;
ERROR: syntax error at or near "COUNT" at character 25
LINE 1: SELECT dom_id, dom_name COUNT(usr_dom_id) AS usr_count FROM ...

I've been struggling with some deadlines, so for now I'm using just:

SELECT d.dom_id, d.dom_name FROM domains d WHERE (SELECT COUNT(*) FROM
users u WHERE u.usr_dom_id=d.dom_id) > 0 ORDER BY d.dom_name ASC;

Which gives me just the domains with at least one user under them,
but not the count. This is not ideal, and I will have to come back to it
next week. In the meantime, any idea what the GROUP BY error is? If not,
I'll read through the docs on 'GROUP'ing once I get this deadline out of
the way.

Thank you all for your help! I am sure I will have more question(s)
next week as soon as I can get back to this.

Madi