2. select s.age,count(*) from s,sc where s.s# = sc.s# group by s.sex
3. select s#,max(grade) from (select s#,avg(grade) as grade from s,sc where s.s# = sc.s# group by s.s#)
4. delete from sc where grade < (select avg(grade) as grade from c,sc where c.cname = 'java' adn c.c# = sc.c#)
5. select s#,grade
from (select s#,avg(grade) as grade from s,sc where s.s# = sc.s# group by s.s#)
where grade>80