Why MYSQL IN keyword not considering NULL values

Question

I am using the following query:

select count(*) from Table1 where CurrentDateTime>\'2012-05-28 15:34:02.403504\' and Error not in (\'Timeout\',\'Connectio         


        

Answer 1

IN returns a trivalent BOOLEAN (which accepts NULL as a value). NOT IN returns the trivalent negation of IN, and negation of NULL is a NULL.

Imagine we have a table with all numbers from 1 to 1,000,000 in id and this query:

SELECT  *
FROM    mytable
WHERE   id IN (1, 2, NULL)

or its equivalent:

SELECT  *
FROM    mytable
WHERE   id = ANY
             (
             SELECT  1
             UNION ALL
             SELECT  2
             UNION ALL
             SELECT  NULL
             )

The predicate returns TRUE for 1 and 2 and NULL for all other values, so 1 and 2 are returned.

In its oppposite:

SELECT  *
FROM    mytable
WHERE   id NOT IN (1, 2, NULL)

, or

SELECT  *
FROM    mytable
WHERE   id <> ALL
             (
             SELECT  1
             UNION ALL
             SELECT  2
             UNION ALL
             SELECT  NULL
             )

, the predicate returns FALSE for 1 and 2 and NULL for all other values, so nothing is returned.

Note that boolean negation not only changes the operator (= to <>), but the quantifier too (ANY to ALL).