Question

What are examples of SQL Server Query features/clauses that should be avoided?

Recently I got to know that the NOT IN clause degrades performance badly.

Do you have more examples?

Answer 1

The reason to avoid NOT IN isn’t really performance, it’s that it has really surprising behaviour when the set contains a null. For example:

select 1
where 1 not in (2,null)

This won’t return any rows, because the where is interpreted like:

where 1 <> 2 and 1 <> null

First 1 <> null evaluates to unknown. Then 1 <> 2 and unknown evaluates to unknown. So you won’t receive any rows.