request help forming query

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.

Can anyone help with this please? Thanks!