CELKO (12/6/2016)
...The original the BIT data type in SQL Server was what computer science majors think it is {0, 1}, that he became a numeric type in all numeric types are NULL-able so it was {null, 0, 1}. I made a couple of bucks of telling people they should of had an is null constraint in their schema....
constraining this type of column to be NOT NULL would seem to eliminate the possibility of representing an unknown value, or are you suggesting that everything should be in a code/lookup type of table, even if the only applicable values are {true, false, unknown}?