From: Joerg Bruehe (Joerg.BrueheSun.COM)
Date: Tue Dec 29 2009 - 03:27:26 CST

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Martijn,

thanks for your excellent mail:

Martijn Tonies wrote:
> [[...]]
>
> A column can have two states: null or not null. It either has
> data (a value, depending on the datatype), or no data (null),
> which is where <column> IS NULL (has no data) or
> <column> IS NOT NULL (has data) comes into play.

To make it more explicit:
The term "null value" is no proper expression in relational theory.
Values can be compared to each other, and they are equal or not, some of
them even are ordered.
"Null" is no value but a state, signalling "value is unknown".
And of two unknowns you can't even tell whether they are equal or not,
so the result of comparing anything (be it a value or "unknown") to
"unknown" is again "unknown".

That's why logic in SQL is three-valued: true, false, and unknown.

Regards,
Jörg

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