How to find rows in one table that have no corresponding row in another table

Question

I have a 1:1 relationship between two tables. I want to find all the rows in table A that don\'t have a corresponding row in table B. I use this query:

SELEC         


        

Answer 1

You can also use exists, since sometimes it's faster than left join. You'd have to benchmark them to figure out which one you want to use.

select
    id
from
    tableA a
where
    not exists
    (select 1 from tableB b where b.id = a.id)

To show that exists can be more efficient than a left join, here's the execution plans of these queries in SQL Server 2008:

left join - total subtree cost: 1.09724:

exists - total subtree cost: 1.07421: