Returning null for joined tables when one column non existant

Hi all,

   I've got a query that crosses a few tables. For example:

SELECT
      a.foo, b.bar, c.baz
FROM
      aaa a, bbb b, ccc c
WHERE
      a.a_id=b.b_a_id AND a.a_id=c.c_a_id AND a.a_id=1;

   Obviously, if there is no match in 'bbb' or 'ccc' then nothing will
be returned, even if there is a match in one or both of the other
tables. Is there a way to say something like 'b.bar OR NULL' to make
sure that the matching columns with data still show and ones without a
match return NULL (or some string)?

Thanks!