Select statement to get the difference

[Stefan Berger]

How to tell it in MySQL
 
 
I have 2 tables and i want to find difference (the id exists in one
table but in the other not) between them. 
 
With SELECT attachment_id from tbl_attachment, tbl_msg2atta where
attachment_id = atta_id
i receive all id wich are equals in both tables.
 
I need to know which id is not present in the other table.   
 
In some other database i can use something like not exists but here in
Mysql i have troubles to deal it.
 
Thanks for hints.
 
Regards