Re: Find users that have ALL categories

On Thu, Jul 01, 2010 at 12:37:55PM +0100, Sam Mason wrote:
> On Thu, Jul 01, 2010 at 04:26:38AM -0700, David Fetter wrote:
> > On Wed, Jun 30, 2010 at 12:11:35AM -0700, Nick wrote:
> > > Is this the most efficient way to write this query? Id like to
> > > get a list of users that have the categories 1, 2, and 3?
> > >
> > > SELECT user_id FROM user_categories WHERE category_id IN (1,2,3)
> > > GROUP BY user_id HAVING COUNT(*) = 3
> >
> > The above method depends on (user_id, category_id) being unique,
> > and excludes users with, say, categories 1, 2, 3 and 4. Are you
> > sure that that latter is what you want?
>
> AFAICT, the above code will include a user with categories 1 to 4.
> Why do you think otherwise?
>
> If the (user_id,category_id) combination isn't unique, it's easy to
> change the HAVING clause into HAVING COUNT(DISTINCT category_id) =
> 3.

Oops. You're right, of course. That's what I get for posting before
waking up. ;)

Cheers,
David.
--
David Fetter <david(at)fetter(dot)org> http://fetter.org/
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