The purpose of EXCEPT is to handle set subtraction. This parallels UNION handling set addition and INTERSECT handling set intersection. Because they are set operators, the require that both sets in the operation have the same shape (in order to satisfy first normal form). They are not equivalent to various joins, although you may be able to achieve the same results with either method. Specifically, EXCEPT is designed to implement set subtraction, not replace the anti semi-join.

Drew