I read this question, decided it was a bit weird because where clauses don't evaluate to anything and concluded that it must be asking what truth-value/values the expression in the where clause evaluates to and ansered accordingly (UNKNOWN). The answer given as correct is FALSE. The explanation simply says that the contained expression always evaluates UNKNOWN, and doen't make any suggestion of how FALSE comes from here. I guess the author has invented a new terminology: in this new terminology a filter evaluates to something instead of including or excluding a row, and "evaluates to false" means "excludes the current row". That's not very useful, as the existing terminology makes perfect sense.