十三、修改表时添加约束

a) 创建主表clazz

create table clazz (

       cno number(3) primary key,

       cname varchar2(20) not null,

       croom number(3)

);

b) 创建从表student

create table student (

       sno number(4),

       sname varchar2(12) not null,

       gender char(3) default '男',

       age number(3),

       sdate date,

       email varchar2(50),

       cno number(3)

);

c) 给student表添加约束

-- 主键约束

alter table student add constraints pk_student primary key (sno);

-- 唯一约束

alter table student add constraints uk_student_email unique (email);

-- 检查约束

alter table student add constraints ck_student_age check (age between 18 and 30);

alter table student add constraints ck_student_gender check (gender in ('男','女'));

-- 外键约束

alter table student add constraints fk_student_cno foreign key (cno) references clazz (cno);