danmcb wrote:
> Hi
>
> !'ve been wondering how to formulate a query to get a set of objects
> out of a database, and am a bit stuck. I hope that someone here might
> be able to help.
>
> This is what the db looks like:
>
> Table TYPES
> id int primary key,
> description text
>
> Table GROUPS
> id int primary key
> description text
>
> Tables WIDGETS
> type_id int references TYPES(id),
> group_id int references GROUPS(id),
> primary key(type_id, group_id)
>
> Now, given two type_id's, say A and B, I would like to find all groups
> (or group_id's of course) that have a widget of both of these two
> types.
>
There must be a more a elegant method but here's the first thing that
came to me:
SELECT group_id FROM widgets WHERE type_id = $1
AND group_id IN (
SELECT DISTINCT group_id FROM widgets WHERE type_id = $2
);
I trust you aren't planning to run this on billions of rows ...
b