实验三-多表查询
1. (简答题) 查询学号为201715121的学生的所选的全部课程的课程名和成绩。
（1）select cname,score
from stc,courses
where stc.cno=courses.cno
and sc.sno='201715121'
（2）select cname,score
from stc NATURAL JOIN courses
where sc.sno='201715121'
（3）select cname,score
from stc JOIN courses on sc.cno=courses.cno
where sc.sno='201715121'2. (简答题) 查询选修计算机导论课程的学生学号和姓名。（要求用where连接实现）
select students.sno,sname
from students,stc,courses
where students.sno=stc.sno
and stc.cno=courses.cno
and cname='计算机导论';3. (简答题)查询选修计算机导论课程的学生学号和姓名（要求用JOIN…ON连接实现）
select students.sno,sname
from students NATURAL join stc
NATURAL join courses
where cname='计算机导论';
或者：
select students.sno,sname
from students join stc on students.sno=stc.sno
join courses on stc.cno=courses.cno
and cname='计算机导论';4. (简答题)查询选修计算机导论课程的学生学号和姓名（要求用嵌套查询实现）
select students.sno,sname
from students
where sno in (
select sno from stc
where cno in (
select cno from courses
where cname='计算机导论' ) );
或者：
select students.sno,sname
from students
where exists (
select * from stc,courses
where stc.cno=courses.cno
and students.sno=stc.sno
and cname='计算机导论') ;5. (简答题) 查询没有选修任何课程的学生信息。
（1）用外连接实现
select students.*
from students left join stc on students.sno=stc.sno
where stc.sno is null;
（2）用嵌套查询实现
select students.*
from students
where sno not in (
select distinct sno
from stc);6. (简答题) 查询至少被三名学生选修的课程号和课程名称。
提示：课程名可能不唯一，但课程号唯一。
select courses.cno,cname,count(*)
from stc join courses on stc.cno=courses.cno
group by courses.cno having count(*)>=3;
group by子句也可写成：
group by courses.cno,cname having count(*)>=3;
7. (简答题)查询同时选修了B001和B002号课程的学生学号。
select distinct sno from stc
where cno='b001'
and sno in (
select sno from stc
where cno='b002' );8. (简答题) 查询同时选修了高等数学和C程序设计课程的学生学号。
select distinct sno
from stc
where cno in ( select cno from courses where cname='计算机导论' )
and sno in ( select sno from stc where cno in (
select cno from courses where cname='高等数学' )
) ;9. (简答题) **查询显示学生的学号、姓名和已经获得的总学分。
提示：
(1)学分=学时/16
(2)选课但没有参加考试或考试不及格的课程，总学分均不统计。
select students.sno,sname,sum(chour)/16
from students,stc,courses
where students.sno=stc.sno
and stc.cno=courses.cno
and score>=60
group by students.sno;10. (简答题)将全部2016级女生的C程序设计课程的成绩每人提高10分，并查看修改后的STC表。
update stc
set score =score+10
where sno in (
select sno from students
where ssex='女' and grade='2016' )
and cno =(select cno from courses --也可以将等号（=）换成in
where cname='C程序设计');
11. (简答题) 将选修“C语言程序设计”的选课记录保存在STC_C表中。
提示：STC_C表尚不存在。
create table stc_c
AS SELECT * FROM stc ---AS后面也可以用连接查询
where cno in (
select cno from courses
where cname='C程序设计');12. (简答题)将选修了“高等数学”课程的选课记录追加到STC_C表中。
insert into stc_c
SELECT * FROM stc
where cno in (
select cno from courses
where cname='高等数学'
)；
13. (简答题)将所有女生的选课成绩置空。
update stc
set score =NULL
where sno in (
select sno from students
where ssex='女');
14. (简答题)删除没有被学生选修的课程信息。
delete from courses
where cno not in (
select cno distinct from stc);