Oracle如何给数据库添加约束过程解析

主键约束(两个特性)

1:主键必须写

2:主键不可重复

create table stu01(
  sid varchar(100),
  sname varchar2(100)
  --constraint PK_stu01_sid primary key (sid)
 )

 --添加约束(主键)
  alter table stu01 add constraint PK_stu01_sid primary key(sid)  ---添加主键约束
  alter table stu01 drop constraint PK_stu01_sid          ---删除主键约束

唯一约束(UNIQUE)

唯一,但是可以为空

create table stu02(
  sid varchar2(100),
  sname varchar2(100),
  constraint PK_stu02_sid primary key(sid),
  constraint UK_stu02_sname unique(sname)
 )

非空约束(not null)

一般简写

create table stu03(
  sid varchar2(100),
  sname varchar2(100) not null,
  constraint PK_stu03_sid primary key(sid),
  constraint UK_stu03_sname unique(sname)
 )

外键约束

先添加学生表、和课程表

create table stu04(
  sid varchar2(100),
  sname varchar2(100) not null,
  constraint PK_stu04_sid primary key(sid),
  constraint UK_stu04_sname unique(sname)
 )

 --创建课程表
 create table course01(
  cid varchar2(100),
  cname varchar2(100) not null,
  constraint PK_course01_cid primary key(cid),
  constraint UK_course01_cname unique(cname)
 )

 --最后添加成绩表,外键约束课程表和学生表
  create table mark02(
  sid varchar2(100),
  cid varchar2(100),
  mark number,
  constraint PK_mark02_sidAndcid primary key(sid,cid),
  constraint FK_mark02_sid foreign key(sid) references stu04(sid),
  constraint FK_mark02_cid foreign key(cid) references course01(cid),
  constraint UK_mark02_mark unique(mark)
  )

 --删除外键约束
 alter table mark01 drop constraint FK_mark02_sid
  alter table mark01 drop constraint FK_mark02_cid