NULL values are excluded. Why?

Question

This is about a bizarre behaviour I found in Microsoft Sql Server. Please correct me if I\'m wrong.

SELECT COUNT(*) FROM TABLEA 
WHERE [Column1] IS NULL;
         


        

Answer 1

SQL uses three valued logic.

t1.[Column1] NOT IN ('Cross/Up sell', 'Renegotiation', 'Renewal') 

is equivalent to

t1.[Column1] <> 'Cross/Up sell' AND  
t1.[Column1] <> 'Renegotiation' AND 
t1.[Column1] <>  'Renewal')

When t1.[Column1] is NULL this expression evaluates to UNKNOWN rather than TRUE so these rows are not returned.

The only time NULL NOT IN ( ... ) will be returned is if the NOT IN clause evaluates to an empty set.