CREATE VIEW SBOY(S#,SNAME,CNAME,GRADE)
AS
SELECT S.S#,S.SNAME,C.CNAME,SC.GRADE
FROM S,C,SC
WHERE S.SEX='M' AND S.S#=SC.S# AND SC.C# = C.C#;


SELECT S#,SNAME
FROM SBOY
WHERE S# IN(SELECT S#
		    FROM SBOY
		    GROUP BY S#
		    HAVING AVG(GRADE)>80);


SELECT COUNT(C#), AVG(GRADE)
FROM SC
GROUP BY S#;


CREATE USER us
IDENTIFIED BY us;
GRANT select
ON S
TO us;

REVOKE select
ON S
FROM us;

GRANT select
ON SC
TO us;

REVOKE select
ON SC
FROM us;

GRANT select
ON C
TO us;

REVOKE select
ON C
FROM us;