X: SELECT [list] FROM Table_Left L INNER JOIN Table_Right R ON L.Key = R.Key ;

Y: SELECT [list] FROM Table_Left L FULL OUTER JOIN Table_Right R ON L.Key = R.Key ;

Z: SELECT [list] FROM Table_Left L FULL OUTER JOIN Table_Right R ON L.Key = R.Key WHERE L.Key IS NULL OR R.Key IS NULL ;

Does this mean X = Y minus Z?