Re: Find users that have ALL categories

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.

--
Sam http://samason.me.uk/