> I have no way of knowing if that means there's a record [5, NULL] in table y, or if there's no record in table y that starts with 5. You can always add SELEC

> I have no way of knowing if that means there's a record [5, NULL] in table y, or if there's no record in table y that starts with 5. You can always add SELECT y.a, which will allow you to disambiguate between the two options (it will be non-NULL in the first case and NULL in the second).