Exactly. I've wondered why no SQL implementation (that I know of) has optional assertions like "join exactly 1 some_table" or "select assert-not-null(some_colum

Exactly. I've wondered why no SQL implementation (that I know of) has optional assertions like "join exactly 1 some_table" or "select assert-not-null(some_column)". I don't see any reason why this would be a performance killer; in fact, it might even be possible to prove and then cache with the query plan.