表结构：

S(SNO,SNAME) 代表 学号。学生姓名；

C(CNO,CNAME,CTEACHER) 代表 课号，课程名称。授课老师

SC(SNO,CNO,SCGRADE) 代表 学号。课号。课程成绩

1 查询没选过“黎明”老师的全部学生姓名。

select s.sname

from s

join sc

on sc.sno=s.sno

join c

on c.cno=s.cno

where c.cteacher<>'黎明'

2 查询出2门以上（包括2门）不及格学生的姓名及平均成绩。

select s.sname,c.a_grade

from

(select a.sno,count(cno)

from

(select sno,cno

from sc

where sc.scgrade<60)

group by a.sno

having count(cno)>1)b

join s

on b.sno=s.sno

join

(select avg(scgrade) a_grade,sno

from sc

group by sno)c

on c.sno=s.sno

3 既学过1号课程又学过2号课程的学生姓名；

select s.sname

from s

join

(select count(*),sno

from

(select * from

sc

where sc.cno in(1,2))

grout by sno

having cout(*)>1)a

on a.sno=s.sno