On: You might as well timestamp it

[Spivak]

With an enum you’d have to check the unset state separate as well.

[crocarneiro → Spivak]

He meant that in some SQL Dialects you cannot check FIELD = NULL, you have to use FIELD IS (NOT) NULL (like Oracle). Therefore you cannot use FIELD inside an IN clause either.

[andix → Spivak]

But with an (NOT NULL) enum/integer/string or however you are representing it, every state is a regular case. Not a special case. Null nearly always needs some special treatment.