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@campbell-lange.net>
<www.campbell-lange.net>