From: | Peter Eisentraut <peter_e(at)gmx(dot)net> |
---|---|
To: | Gabor Csuri <gcsuri(at)coder(dot)hu> |
Cc: | PostgreSQL Development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: I don't understand... |
Date: | 2001-05-22 14:40:55 |
Message-ID: | Pine.LNX.4.30.0105221637010.757-100000@peter.localdomain |
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Lists: | pgsql-hackers |
Gabor Csuri writes:
> SELECT DISTINCT h_name
> >FROM haszon
> WHERE h_name NOT IN (SELECT cn_name FROM carname)
>
> +--------+
> | h_name |
> +--------+
> +--------+
> Query OK, 0 rows in set (0,10 sec)
>
> Why ?
Because one of the cn_name values is NULL. Observe the semantics of the
IN operator if the set contains a NULL value:
h_name NOT IN (a, b, c)
NOT (h_name = a OR h_name = b OR h_name = c)
Say c is null:
NOT (h_name = a OR h_name = b OR h_name = NULL)
NOT (h_name = a OR h_name = b OR NULL)
NOT (NULL)
NULL
which is false.
You might want to add a ... WHERE cn_name IS NOT NULL in the subquery.
--
Peter Eisentraut peter_e(at)gmx(dot)net http://funkturm.homeip.net/~peter
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