oracle 约束

约束是表中列的属性，用来维护数据结构完整性的一种手段

约束的种类：

NOT NULL

UNIQUE

PARIAMRY KEY

FOREIGN KEY

CHECK

 

enble validate  检查现有数据和新数据是否符合约束

enable novlidate  
检查新数据是否符合约束

disable validate   给表加锁

disable novalidate 

创建表的时候带有约束信息:

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

 

查看约束信息:

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能在表级别指定！

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;

 

测试数据依赖关系:

delete dept where deptno=40;

 

删除约束:

alter table e drop constraint fk_e_deptno;

 

外键约束的两种特殊情况（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;

 

约束的状态:

select constraint_name,

status

from user_constraints

where table_name='E';

 

禁止约束:

alter table e modify constraint UK_E_EMAIL disable;

 

启用约束:

alter table e modify constraint UK_E_EMAIL enable;

 

删除外键，重新建立父子关系(子表为e，父表为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);

 

父子关系建立后对父表结构修改的影响:

alter table d drop (deptno);

ORA-12992: cannot drop parent key column

 

强制删除父表主键:

alter table d drop (deptno) cascade constraints;