sql约束（constraint）

约束

constraint
约束条件(数据库中的对象)
类型5种
1.非空约束:not null
2.唯一约束:unique
3.主键(语法:非空唯一,逻辑:单独一条记录):primary key
4.外键(被参考的字段必须是主键):
5.check:

create table stu
(
id number(6),
name varchar2(20) constraint stu_name_nn not null,
sex number(2),
age number(3),
sdate date,
grade number(2) default 1,
class number(4),
email varchar2(50) unique
)

--------------------------------------------------
create table stu
(
id number(6),
name varchar2(20) constraint stu_name_nn not null,
sex number(2),
age number(3),
sdate date,
grade number(2) default 1,
class number(4),
email varchar2(50),
--email,name组合不能重复约束
constraint stu_name_email_uni unique(email,name)
)
---------------------------------------------------
create table stu
(
id number(6),
name varchar2(20) constraint stu_name_nn not null,
sex number(2),
age number(3),
sdate date,
grade number(2) default 1,
class number(4) ,
email varchar2(50),
--email,name组合不能重复约束
constraint stu_name_email_uni unique(email,name),
constraint stu_id_pk primary key(id) ,
constraint stu_class_fk foreign key key (class) references class(id)
)
---------------------------------------------------
create table class
(
id number(4) primary key,
name varchar2(20) not null,
)

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