Re: String Comparison and NULL

On Mon, 28 Apr 2008 seijin(at)gmail(dot)com wrote:

> 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?

Comparisons against null with =, <> and so on return unknown not true or
false and WHERE clauses only return rows where the condition is true. You
might want to read up on the ternary (three valued) logic and nulls. I
haven't read through it but the wikipedia page on null is pretty long.

http://en.wikipedia.org/wiki/Null_%28SQL%29