Question

How can I write an efficient SQL query that returns columns form table A plus a bit column that specifies whether the PK of table A exists in table B? I am using MS SQLServer 2005. Thanks.

Answer 1

It depends if the foreign table relationship is 1:1 or 1:Many. If is 1:1 the you can use a normal join:

select A.*, case when b.id is null then 0 else 1 end
from A 
left join B on A.id = B.id;

If the relation is 1:Many then the join would multiply the result, so you have to restrict it, and there are several ways. A simple way is using outer apply and top:

select A.*, case when b.id is null then 1 else 0 end
from A
outer apply (
 select top (1) id from B where A.id = B.id) as b;

As for performance, almost all solution proposed will perform the same, provided there is an appropriate index on B(id) in place.