Using parent row in subquery where clause

[leinad31]

Hopefully you can see from the code what I want to achieve but am currently unsuccesful.

I want to join on to each booking the latest record from from the audit table for that booking, however because the audit table is so big I would like to limit the nested select to only search for the records corresponding to the booking on each join iteration.

Currently though the statement does not recognise b as a table alias.

SELECT * FROM
Bookings as b
INNER JOIN 		
		(SELECT MAX(record_id) as LatestAuditID, bookingnumber 
		FROM audit a 
		WHERE a.bookingnumber=b.booking_number 
		GROUP BY bookingnumber
		) AS aud 
		ON aud.bookingnumber = b.booking_number

Since the audit table is big we have to avoid full table scans... what are the available indices on audit table and bookings table? How many rows are in audit table and in bookings table?