约束是表中列的属性，用来维护数据构造残缺性的一种足段
约束的种类：
NOT NULL
UNIQUE
PARIAMRY KEY
FOREIGN KEY
CHECK

enble validate  搜检现稀有据战新数据可否适宜约束
enable novlidate   搜检新数据可否适宜约束
disable validate   给表加锁
disable novalidate 

创立表的时辰带有约束信息:
create table e (empno number(4) constraint pk_e_empno（约束名） primary key,
ename varchar2(10) not null, not null只能在列级别
email varchar2(30),
constraint uk_e_email unique (email)); 约束写在后面为表级别约束，要指定列名

查察约束信息:
select constraint_name,
constraint_type,
SEARCH_CONDITION,
R_CONSTRAINT_NAME
from user_constraints
where table_name="E";

select constraint_name,column_name from user_cons_columns
where table_name="E";

添加约束:
alter table e add (sal number(7,2),deptno number(2));

alter table e add constraint ck_e_sal check (sal>999 and sal is not null);

alter table e add constraint fk_e_deptno foreign key(deptno) references dept (deptno);

注意:not null 约束只能在列级别指定不克不及在表级别指定！
alter table e modify (ename varchar2(10) constraint nn_e_ename not null);

select table_name,constraint_type from user_constraints where constraint_name="PK_DEPT";

select table_name,column_name from user_cons_columns
where constraint_name="PK_DEPT";

insert into e values (1,"X1","x1@uplooking.com",1000,10);
insert into e values (2,"X2","x2@uplooking.com",2000,20);
insert into e values (3,"X3","x3@uplooking.com",2000,40);
commit;

测试数据依好关系:
delete dept where deptno=40;

删除约束:
alter table e drop constraint fk_e_deptno;

中键约束的两种不凡情形（主键删除有级联）
alter table e add constraint fk_e_deptno foreign key(deptno) references dept (deptno) on delete set null;

alter table e add constraint fk_e_deptno foreign key(deptno) references dept (deptno) on delete cascade;

约束的外形:
select constraint_name,
status
from user_constraints
where table_name="E";

胁制约束:
alter table e modify constraint UK_E_EMAIL disable;

启用约束:
alter table e modify constraint UK_E_EMAIL enable;

删除中键，从头成立女子关系(子表为e，女表为d，d表拷贝dept表数据):
alter table e drop constraint fk_e_deptno;

create table d as select * from dept;

alter table d add constraint pk_d_deptno primary key (deptno);

alter table e add constraint fk_e_deptno foreign key(deptno) references d (deptno);

女子关系成立后对女表构造修改的影响:
alter table d drop (deptno);
ORA-12992: cannot drop parent key column

强制删除女表主键:
alter table d drop (deptno) cascade constraints;