Question

I’m trying to clean up some data in SQL server and add a foreign key between the two tables.

I have a large quantity of orphaned rows in one of the tables that I would like to delete. I don’t know why the following query would return 0 rows in MS SQL server.

–This Query returns no Rows

select * from tbl_A where ID not in ( select distinct ID from tbl_B
)

When I include IS NOT NULL in the subquery I get the results that I expect.

— Rows are returned that contain all of the records in tbl_A but Not in tbl_B

select * from tbl_A where ID not in ( select distinct ID from tbl_B
where ID is not null )

The ID column is nullable and does contain null values. IF I run just the subquery I get the exact same results except the first query returns one extra NULL row as expected.

Answer 1

This is the expected behavior of the NOT IN subquery. When a subquery returns a single null value NOT IN will not match any rows.

If you don’t exclusively want to do a null check, then you will want to use NOT EXISTS:

select * 
from tbl_A A
where not exists (select distinct ID 
                  from tbl_B b
                  where a.id = b.id)

As to why the NOT IN is causing issues, here are some posts that discuss it:

NOT IN vs. NOT EXISTS vs. LEFT JOIN / IS NULL

NOT EXISTS vs NOT IN

What's the difference between NOT EXISTS vs. NOT IN vs. LEFT JOIN WHERE IS NULL?