I just covered how comparisons with NULLs return more
NULLs, so how do you test for NULL? There are actually two clauses designed specifically to compare NULLs.

The first is the IS NULL comparison clause. It is a clause rather than an operator because I mentioned
earlier that operators on NULLs return NULLs. Here
is how it works:

SELECT NULL = NULL;
 ?column?
----------
 (null)
 
SELECT NULL IS NULL;
 ?column?
----------
 t
 
SELECT NULL IS NOT NULL;
 ?column?
----------
 f
 
SELECT * FROM inctest;
   x
--------
     30
     40
 (null)
 
SELECT * FROM inctest WHERE x IS NULL;
   x
--------
 (null)
 
SELECT * FROM inctest WHERE x IS NOT NULL;
 x
----
 30
 40

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