query problem - get count in related table

Thanks to everyone on the list I'm up and running with postgres.

I'm having trouble doing a query:

table a | table b
------------------------------
id data | id
------------------------------
1 2 | 2
2 1 | 2
3 4 | 1
| 1
| 1
| 3

I'd like to run a query that returned the following results:

a.id a.data count(b.id where b.id = a.data)
---------------------------------------------
1 2 2
2 1 3
3 4 0

Thanks for any help.
Rory

--
Rory Campbell-Lange
<rory(at)campbell-lange(dot)net>
<www.campbell-lange.net>