Restricting a LEFT JOIN

Question

I have a table, let\'s call it \"a\" that is used in a left join in a view that involves a lot of tables. However, I only want to return rows of \"a\" if they also join wit

Answer 1

    Select ... 
    From Main 
     Left Join a on main.col2 = a.col2
    where a.col3 in (select b.col3 from b) or a.col3 is null

you may also need to do some indexing on a.col3 and b.col3