您好，这样：

--1

select Sname,Sage from Student where Sage<(select Sage from Student where Sno='某一学生') and Sdept='数学系'

--2

select Sname from Student where Sno in (select Sno from SC)

--3

select Sname from Student where Sno in (select Sno from SC group by Sno having count(*)=(select count(*) from Course ))

--4

题意不清楚。