Re: When is a record NULL?

Joshua Tolley <eggyknap(at)gmail(dot)com> writes:
> On Thu, Jul 23, 2009 at 06:46:25PM -0700, David E. Wheeler
>> Yes, but given that the standard says that `ROW(1, NULL)` is NULL, then I
>> would expect it to be NOT DISTINCT from `ROW(2, NULL)`.

> Wait, didn't we decide upthread that the standard said ROW(1, NULL) isn't
> NULL?

David misspoke in the quoted statement, as I believe he figured out soon
thereafter. For that row value, neither IS NULL nor IS NOT NULL will
return true. The spec defines them in such a way that they are not inverses
for row values.

SQL2008 points out:

NOTE 219 - For all R, "R IS NOT NULL" has the same result as
"NOT R IS NULL" if and only if R is of degree 1. Table 14,
"<null predicate> semantics", specifies this behavior.

That table looks like this:

R IS R IS NOT NOT R IS NOT R IS NOT
_Expression_______NULL____NULL__________NULL__________NULL_________

| degree 1: null | true_ | false_ | false_ | true_ |
| | | | | |
| degree 1: not | false_| true_ | true_ | false_ |
null

| degree > 1: | true_ | false_ | false_ | true_ |
| all null | | | | |
| | | | | |
| degree > 1: | false_| false_ | true_ | true_ |
| some null | | | | |
| | | | | |
| degree > 1: | false_| true_ | true_ | false_ |
|_none_null______|_______|_____________|____________|______________|

regards, tom lane