create table teacher
(
tid int(10) primary key auto_increment,
tname varchar(20),
tage int(10)
);
use studb;

create table student
(
sid int(10) primary key auto_increment,
sname varchar(20),
sage int(10),
tid int(10) REFERENCES teacher(tid) 
);

--select teacher.tname from teacher,student where student.sname = '张三'
select t.tname from teacher t,student s where s.sname = '张三' and t.tid = s.tid

create table course
(
cid int(10) primary key,
cname varchar(20),
tid int(10) REFERENCES teacher(tid)
);

create table sc
(
scid int(10) primary key,
sid int(10) REFERENCES student(sid),
cid int(10) REFERENCES course(cid),
score int(10)
);

select c.cname from course c,student s,sc where s.sname = '小张' 
and s.sid = sc.sid and c.cid = sc.cid;

select sname from student s,course c,sc where c.cname='android' and sc.score>=60
and s.sid = sc.sid and c.cid = sc.cid;

delete from sc where sid = (select sid from student where sname = '小张'); 

update sc set score=score+5 where cid=????;

select tid from teacher where tname='李老师' ==1
select cname from course where tid = 1 ==课程名字，李老师教的
select cid from course where cname='android' ==课程ID
update sc set score=score+5 where cid=
(
select cid from course where cname=
(
select cname from course where tid =
(
select tid from teacher where tname='李老师'
)
)
);