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@',1000,10);
insert into e values (2,'X2','x2@',2000,20);
insert into e values (3,'X3','x3@',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;
