String Comparison and NULL

From: seijin(at)gmail(dot)com
To: pgsql-general(at)postgresql(dot)org
Subject: String Comparison and NULL
Date: 2008-04-28 16:39:24
Message-ID: 42dbc725-f6b0-471e-8a88-2a93d8cea542@y38g2000hsy.googlegroups.com
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Hi,

I'm fairly new to PG and databases in general so this may very well be
a problem in my thought process.

If I have a simple table with an ID (integer) and Animal (text) like
this...

1 Dog
2 Cat
3 NULL
4 Horse
5 Pig
6 Cat
7 Cat

... and I do something like "select id where animal <> 'Cat';" then
shouldn't 1, 3, 4 and 5 be picked? As it is I only get 1, 4 and 5.
NULL is not 'Cat'. I realize that if I were testing for NULL itself I
would use IS or IS NOT but this...? I'm a little confused.

Thanks!

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