> Aaaargh! No, it doesn't. It means NULL. Nothing else.
Well, x = UNKNOWN doesn't make any sense... the answer is UNKNOWN.
x IS UNKNOWN does make sense, the answer is true or false. Replace
UNKNOWN with NULL...
Actually it means what the DBA wants it to mean (which opens the door to
many a misguided design...)
I hereby light a candle to the pgsql designers who didn't inflict
00-00-0000 00:00:00 upon us besides NULL dates.