判断子表外键约束参数类型

判断子表外键约束参数类型

 

 

前言：在存在主外键约束的父子表关系中，删除父表的数据，子表的数据需要首先删除，否则报错

#在外键约束的参数中有三种模式：默认无配置NO ACTION ，级联删除 ON DELETE CASCADE，父表删除的相关数据，子表外键约束列对应的数据置为null

 

#如何查询外键约束类型是什么参数

 

#直接删除父表的记录测试：

delete scott.dept where deptno=20

*

ERROR at line 1:

ORA-02292: integrity constraint (SCOTT.FK_DEPTNO) violated - child record found

#子表还存在数据，且外键约束类型为默认，因此删除父表数据，需要手工提取删除子表的相关记录

 

#查询子表的外键约束类型

SCOTT >  select OWNER,CONSTRAINT_NAME,CONSTRAINT_TYPE,TABLE_NAME,R_OWNER,R_CONSTRAINT_NAME,DELETE_RULE from USER_CONSTRAINTS where CONSTRAINT_TYPE='R'

 

OWNER      CONSTRAINT CONSTRAINT TABLE_NAME R_OWNER    R_CONSTRAI DELETE_RUL

---------- ---------- ---------- ---------- ---------- ---------- ----------

SCOTT      FK_DEPTNO  R          EMP        SCOTT      PK_DEPT    NO ACTION

 

#添加一个外键约束：为 on delete cascade参数

SCOTT > alter table emp add constraint fk_deptno foreign key(deptno)  references dept(deptno) on delete cascade;

 

#查询

SCOTT > select OWNER,CONSTRAINT_NAME,CONSTRAINT_TYPE,TABLE_NAME,R_OWNER,R_CONSTRAINT_NAME,DELETE_RULE from USER_CONSTRAINTS where CONSTRAINT_TYPE='R';

 

OWNER      CONSTRAINT CONSTRAINT TABLE_NAME R_OWNER    R_CONSTRAI DELETE_RUL

---------- ---------- ---------- ---------- ---------- ---------- ----------

SCOTT      FK_DEPTNO  R          EMP        SCOTT      PK_DEPT    CASCADE

#对比：从视图： user_constraints视图中的 delete_rule列可以看出外键约束的参数类型

 

#测试 on delete set null参数现象：

SYS > create table hr.emp as select * from scott.emp;

SYS > grant references on  scott.dept to hr;

 

SYS > alter table hr.emp add constraint fk_deptno foreign key (deptno) references scott.dept(deptno) novalidate;

SYS > alter table hr.emp drop constraint fk_deptno;

 

SYS > alter table hr.emp add constraint fk_deptno foreign key (deptno) references scott.dept(deptno) on delete set null;

SYS > delete scott.dept where deptno=20;

1 row deleted.

SYS > select * from hr.emp where deptno=20;

no rows selected

SYS >  select * from hr.emp;

     EMPNO ENAME      JOB       MGR HIREDATE          SAL      COMM    DEPTNO

---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------

      7900 JAMES      CLERK       7698 1981-12-03 00:00:00    950                    30

      7902 FORD       ANALYST    7566 1981-12-03 00:00:00    5000

 

#外键约束参数，会导致对父表delete操作需要考虑子表的数据，了解外键约束三种参数，默认不处理，delete级联删除子表数据，set null对子表复合的外键列置为Null,可以有助于数据处理的思考