I'd check to see if y.a IS NULL in that situation. I'm sure there are cases where it matters, but most of the time for me the difference between "there's a row

I'd check to see if y.a IS NULL in that situation. I'm sure there are cases where it matters, but most of the time for me the difference between "there's a row in y, but the value is NULL" and "there's no row in y, the value is NULL" is irrelevant. I can't think of a time when that distinction has been important, and I'm working on a project converting thousands of complex SQL queries. The thing that really bugs me about NULL is the default assumptions - 99.9% of the time I want NULL in a string context to be the empty string and NULL in a numeric context to be 0, but I have to use ISNULL or COALESCE to get that. I wish it were the other way round where NULL is treated as '' or 0 by default, but I can do something special if I really want to test for NULL'ness.