Re: not quite expected behaviour when using IN clause

On Fri, 30 Sep 2005, Joe Maldonado wrote:

> Hello all,
>
> I apologize for the wide distribution but we recently ran into an
> interesting behaviour using PostgreSQL 8.0.3 and did not know whether this
> was a bug or intended behaviour.
>
> When an IN clause contains a NULL value the entire in clause is considered
> as being false, thus no records are returned.
>
> Why doesn't IN evaluate NULL as a value?
>
> so for example:
>
> SELECT count(*) FROM test WHERE key NOT IN ('something');
> returns the count of rows...
>
> where
> SELECT count(*) FROM test WHERE key NOT IN ('something', NULL);
> does not. table test does not have any NULL values in the key column.

RVC NOT IN IPV is described as NOT(RVC IN IPV) which turns into
NOT(RVC = ANY IPV)

= ANY does the following:
c) If the implied <comparison predicate> is true for at least
one row RT in T, then "R <comp op> <some> T" is true.

d) If T is empty or if the implied <comparison predicate> is
false for every row RT in T, then "R <comp op> <some> T" is
false.

e) If "R <comp op> <quantifier> T" is neither true nor false,
then it is unknown.

So, for key NOT IN ('something', NULL) there are two cases,
key = 'something', in which case c applies and IN would be true and NOT
IN false so the row doesn't get returned
key <> 'something', in which case key = 'something' is false and key=NULL
is unknown, so e applies and IN is unknown and NOT IN is unknown so the
row doesn't get returned.