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From: Jack Douglas <jack@douglastechnology.co.uk>
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Subject: Re: boolean states
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> I'd vote for just ripping out the:
> |  A third state, "unknown", is represented by the SQL null value.
>
> sentence entirely. I see no reason why NULL should be talked about in
> particular on the page about boolean data types; there are many data
> types, any of which might be NULL.
NULL is not unique to boolean, but UNKNOWN is - it would surely be wrong=20
to have no mention of it at all on this page. This is because the=20
boolean type is the only one used to represent truth (or logical)=20
values. One of the comments from the link you provided:

> What=92s even more interesting is that for BOOLEAN they invented the=20
> keyword UNKNOWN and the 2003 standard states =93The null value of the=20
> boolean data type is equivalent to the Unknown truth value.=94 So for=20
> BOOLEAN (and only BOOLEAN AFAICT) you=92re supposed to say WHERE=20
> <boolean primary> IS [NOT] UNKNOWN. And in the definition of=20
> =93literal=94, which is supposed to =93Specify a non-null value=94, =93=
boolean=20
> literal=94 is equated to TRUE, FALSE or UNKNOWN (but the latter is=20
> equivalent to a =93null value=94 a few pages later).