Re: [HACKERS] A small type extension example for the contrib dire ctory (fwd)

I wrote:
>>> In the second query, the first two rows have been grouped, but
shouldn't
>>> they not be since b is NULL?  I thought that NULL != NULL?
>
>Note that:
>NULL <> NULL		is false
>NULL = NULL			is false

This was wrong, I digged up some more docu and found:
<something> op NULL		is unknown    note that there exists a
third boolean state (name it ?)
`Unknown values occur when part of an expression that uses an arithmetic
operator is null.`
If a whole where expression evaluates to unknown the row is not chosen.
not:	t -> f, f -> t, ? -> ?
and:	? and t -> ?, ? and f -> f, ? and ? -> ?
or:	t or ? -> t, f or ? -> ?, ? or ? -> ?

Order by: `Null values are ordered as less than values that are not
null.` 
but
	NULL {>|<|<=|>=}  value		is unknown

Andreas