创建数据表

第一个

create table  stu(
sno int auto_increment,
snname varchar(20),
ssex varchar(10),
cno int,
primary key(sno)
);

　　

第二个

 

create table  class(
cno int auto_increment,
sname varchar(20),
primary key(cno)
);

　　

 第三个

create table  score(
id int auto_increment,
cno varchar(20),
degree varchar(10),
primary keyo(id)
);

　

插入数据

insert into 表名(字段名1，字段名2) values (数据1，数据2);

　　

1.找出学习oracle的学生信息

 select * from stu as s join score as o on s.sno = o.sno join class as c on c.cno = o.cno where c.cname = 'oracle';

　　

2.查出姜正国的信息和所学专业

 select c.cname from stu as s join score as o on s.sno = o.sno join class as c on c.cno = o.cno where s.sname = 'jiangzhengguo';

　　

3找出只学了一门的同学信息

select * from stu where sno=(select cno from score group by cno having count(*)=1);

　　

4找出学了三门以上的同学信息

select * from stu where sno=(select cno from score group by cno having count(*)>=3);