CREATE TABLE Student (StuID VARCHAR2, sex VARCHAR2, sportname VARCHAR2)
CREATE TABLE Sportsinfo (StuID VARCHAR2, sex VARCHAR2, sportname VARCHAR2)

Show all male student ids who don't play football.
SELECT StuID FROM Student WHERE sex = 'M' EXCEPT SELECT StuID FROM Sportsinfo WHERE sportname = "Football"
SELECT "StuID" FROM "Student" WHERE "sex" = 'M' EXCEPT SELECT "StuID" FROM "Sportsinfo" WHERE "Football" = "sportname"