SQL & PL/SQL

Hello,

I'm having a bit of head-block on this.

I have 2 tables a and b where a.id is in b 0, 1 or many times.

All I'm wondering is if there is a more efficient way of saying

SELECT DISTINCT a.*, b.a_id
FROM a
LEFT JOIN b ON b.a_id = a.id

given that a is really a complex sql statement on its own.

All I really want is a column which says whether a.id exists in b but which is the most efficient way of doing it,

Regards,

Iain