>>>>> "Chris" == Chris Wilson <chris.wilson@cantabcapital.com> writes:
Chris> However, if we try to invert it by using the != operator, then
Chris> we get unexpected results:
Mr. De Morgan would like a word.
https://en.wikipedia.org/wiki/De_Morgan%27s_laws
In short, if you have a condition of the form (a OR b) and you want to
negate it, then you find that:
NOT (a OR b) is equivalent to (NOT a) AND (NOT b)
Since x = ANY (array[1,2]) is equivalent to (x = 1) OR (x = 2), then
the negation would be (x != 1) AND (x != 2), not OR.
Which can be conveniently expressed as x != ALL (array[1,2]).
So just as you interchange AND and OR when inverting the sense of a
condition, you also interchange ALL and ANY for exactly the same
reasons.
Chris> expression NOT IN (subquery)
https://wiki.postgresql.org/wiki/Don%27t_Do_This#Don.27t_use_NOT_IN
Chris> And is it a bug that one can't use unnest in a NOT IN expression
Chris> in the WHERE clause?
No.
--
Andrew (irc:RhodiumToad)
