十二、约束

一、表格的约束

constraints, 约束, 用于对表格的数据进行限制, 保证表格数据的完整性和一致性.

语法:

constraints 约束名 约束类型 (约束字段)

二、主键约束(primary key)

主键是用于唯一标识一条记录的字段, 必须保证既非空又唯一.

一张表中, 只能有一个主键

a) 在表级别定义主键约束, 对约束统一管理

create table student (

       sno number(4),

       sname varchar2(12),

       gender char(3),

       age number(3),

       sdate date,

       clazz varchar2(20),

       email varchar2(50),

       constraints pk_student primary key (sno)

);

b) 表级别简化版定义主键约束, 省略约束名

create table student (

       sno number(4),

       sname varchar2(12),

       gender char(3),

       age number(3),

       sdate date,

       clazz varchar2(20),

       email varchar2(50),

       primary key (sno)

);

c) 在列级别定义主键约束

create table student (

       sno number(4) constraints pk_student primary key,

       sname varchar2(12),

       gender char(3),

       age number(3),

       sdate date,

       clazz varchar2(20),

       email varchar2(50)

);

d) 在列级别简化版定义主键约束

create table student (

       sno number(4) primary key,

       sname varchar2(12),

       gender char(3),

       age number(3),

       sdate date,

       clazz varchar2(20),

       email varchar2(50)

);

e) 联合主键, 只能在表级别定义, 因为一个表只能有一个主键

create table student (

       sno number(4),

       sname varchar2(12),

       gender char(3),

       age number(3),

       sdate date,

       clazz varchar2(20),

       email varchar2(50),

       constraints pk_student primary key (sno, email)

);

二、非空约束(not null)

字段值不允许为空, 非空约束只能在列级别定义

create table student (

       sno number(4),

       sname varchar2(12) not null,

       gender char(3),

       age number(3),

       sdate date,

       clazz varchar2(20),

       email varchar2(50),

       constraints pk_student primary key (sno)

);

三、唯一约束(unique)

create table student (

       sno number(4),

       sname varchar2(12) not null,

       gender char(3),

       age number(3),

       sdate date,

       clazz varchar2(20),

       email varchar2(50), -- unique,

       constraints pk_student primary key (sno),

       constraints uk_student_email unique (email)

);

四、检查约束(check)

用于限定字段值的取值范围

create table student (

       sno number(4),

       sname varchar2(12) not null,

       gender char(3) check (gender in ('男', '女')),

       age number(3),

       sdate date,

       clazz varchar2(20),

       email varchar2(50),

       constraints pk_student primary key (sno),

       constraints uk_student_email unique (email),

       constraints ck_student_age check (age between 18 and 30)

);

五、外键约束(foreign key)

用于约束表和表之间的关系. 两张表的依赖关系. 以emp表和dept表为例, emp表依赖dept表. 因此, dept表可以被称之为主表, emp表被称之为从表.

注意:

• Ø 主表中, 只有主键或者唯一键才可以被从表参考.
• Ø 从表中作为外键的列, 类型一定要和主表的被参考列相同.

a) 建立主表 clazz

create table clazz (

       cno number(3) primary key,

       cname varchar2(20) not null,

       croom number(3)

);

b) 创建从表, 并定义外键 student

create table student (

       sno number(4),

       sname varchar2(12) not null,

       gender char(3) default '男' check (gender in ('男', '女')),

       age number(3),

       sdate date,

       email varchar2(50),

       cno number(3),

       constraints pk_student primary key (sno),

       constraints uk_student_email unique (email),

       constraints ck_student_age check (age between 18 and 30),

       constraints fk_student_cno foreign key (cno) references clazz (cno)

);

c) 也可以在列级别定义外键, 如下:

create table student (

       sno number(4),

       sname varchar2(12) not null,

       gender char(3) default '男' check (gender in ('男', '女')),

       age number(3),

       sdate date,

       email varchar2(50),

       cno number(3) references clazz(cno),

       constraints pk_student primary key (sno),

       constraints uk_student_email unique (email),

       constraints ck_student_age check (age between 18 and 30)

);