Strange results from NOT IN subquery

Question

I have this simple query:

SELECT COUNT(SalesOrderId)
FROM SalesOrder

It gives a result of 14000

Then I have

SELECT COUN         


        

Answer 1

NOT IN with NULL could be tricky.

Let's unwrap it:

col IN (1,2,3, NULL)
<=>
(col = 1) OR (col = 2) OR (col = 3) OR (col = NULL)

For now everything is ok. But it isn't. We cannot compare directly value to NULL because the result is unknown.

Let's check negated condition:

col NOT IN (1,2,3,NULL)
<=>
(col != 1) AND (col != 2) AND (col != 3) AND (col != NULL) 
                                              -- here is the problem

LiveDemo

To sum up last condition is always not true. That is why you get 0 records.

---

When you use NOT IN make sure that you do not compare with NULL value.