--关于主外键的练习


--添加和删除主键与外键语法
--删除外键 语法：alter table 表名 drop constraint 外键约束名 

--添加外键 语法：alter table 表名 add constraint 外键约束名 foreign key(列名) references 引用外键表(列名) 

--删除主键 语法：alter table 表名 drop constraint 主键约束名 

--增加主键 语法：alter 表名 add constraint 主键约束名 primary key(列名) 
--删除主键
alter table sc drop constraint pk_sc
--删除外键
alter table sc drop constraint fk_student
alter table sc drop constraint fk_course


--创建学生表
create table student(

Id int identity(1,1) Primary key,
name varchar(12),
age Int,
sex char(1),
class varchar(6)
);

insert into student values(‘p0001‘,‘王军‘,21,1,‘c101‘); -- (‘p0001‘,‘王军’,20,1,‘c101‘);
insert into student values(‘p0002‘,‘张宇‘,21,1,‘c102‘);  --(‘p0002‘,‘张宇’,21,1,‘cl01‘);
insert into student values(‘p0003‘,‘刘飞‘,22,1,‘c103‘);
INSERT INTO student VALUES(‘p0004‘,‘赵燕‘,18,0,‘c103‘);
INSERT INTO student VALUES(‘p0005‘,‘曾婷‘,19,0,‘c103‘);
INSERT INTO student VALUES(‘p0006‘,‘周慧‘,21,0,‘c104‘);
INSERT INTO student VALUES(‘p0007‘,‘小红‘,23,0,‘c104‘);
INSERT INTO student VALUES(‘p0008‘,‘杨晓‘,18,0,‘c104‘);
INSERT INTO student VALUES(‘p0009‘,‘李杰‘,20,1,‘c105‘);
INSERT INTO student VALUES(‘p0010‘,‘张良‘,22,1,‘c105‘);

--创建课程表
create table course(
id int identity(1,1) Primary key,  --identity(1,1) 从1开始,每次自增1
sname varchar(12) ,
credit numeric(2,1), --数字类型，总长度2  = 整数长度+小数长度。保留1位小数。
teacher varchar(12)
);

insert into course values(‘c001‘,‘Java‘,3.5,‘李老师‘);
insert into course values(‘c002‘,‘高等数学‘,5.0,‘赵老师‘);
insert into course values(‘c003‘,‘JavaScript‘,3.5,‘王老师‘);
INSERT INTO course VALUES(‘C004‘,‘离散数学‘,3.5,‘卜老师‘);
INSERT INTO course VALUES(‘C005‘,‘数据库‘,3.5,‘廖老师‘);
INSERT INTO course VALUES(‘C006‘,‘操作系统‘,3.5,‘张老师‘);


--创建选修表
--选修表就是关系表或中间表，提供了学生表到课程表的映射关系
create Table sc(
sid varchar(10) not null,
cid varchar(10) not null
);


alter table sc Add constraint pk_sc Primary key(sid,cid); --添加主键组
alter table sc add constraint fk_student Foreign key(sid) references student(id); --添加外键约束
alter table sc add constraint fk_course Foreign key(cid) references course(id);--添加外键约束

insert into sc values(‘p0001‘,‘c001‘);
insert into sc values(‘p0001‘,‘c002‘);
insert into sc values(‘p0001‘,‘c003‘);
insert into sc values(‘p0002‘,‘c001‘);
insert into sc values(‘p0002‘,‘c004‘);
INSERT INTO sc VALUES(‘p0003‘,‘c002‘);
INSERT INTO sc VALUES(‘p0003‘,‘c005‘);
INSERT INTO sc VALUES(‘p0004‘,‘C003‘);
INSERT INTO sc VALUES(‘p0005‘,‘C001‘);
INSERT INTO sc VALUES(‘p0006‘,‘C004‘);
INSERT INTO sc VALUES(‘p0007‘,‘C002‘);
INSERT INTO sc VALUES(‘p0008‘,‘C003‘);
INSERT INTO sc VALUES(‘p0009‘,‘C001‘);
INSERT INTO sc VALUES(‘p0009‘,‘C005‘);



--查询没有选课的学生信息和无学生选的课程信息

select *from student where student.id not in(select sid from sc);
select *from course where course.id not in (select cid from sc);

SELECT student.* FROM student LEFT JOIN sc ON student.id=sc.sid LEFT JOIN course ON course.id = sc.cid WHERE course.sname IS NULL;
SELECT course.* FROM course LEFT JOIN sc ON course.id=sc.cid LEFT JOIN student ON student.id = sc.sid WHERE student.sname IS NULL;
--删除表
drop table student;
drop table course;
drop table sc;

--以上练习材料来自互联网