racle使用外键来限制子表中参考的字段值，要求子表中的数据必须在主表中存在。当主表的记录发生变化时导致外键参考唯一约束值发生了变化时，Oracle指定了三种动作：默认值（类似于restrict）、delete cascade和delete set null。(
1.创建父表并初始化数据
SQL> create table t_parent (parent_id int primary key, name varchar2(10));Table created.SQL> insert into t_parent values (1,'record1');1 row created.SQL> insert into t_parent values (2,'record2');1 row created.SQL> insert into t_parent values (3,'record3');1 row created.SQL> commit;Commit complete.
2.创建三种类型的子表t_child1、t_child2和t_child3
(1)no action类别
SQL> create table t_child1 (child1_id int primary key, parent_id int);Table created.SQL> alter table t_child1 add constraint FK_t_child1 foreign key (parent_id) references t_parent (parent_id);Table altered.SQL> insert into t_child1 values (1,1);1 row created.SQL> commit;Commit complete.
(2)cascade类型
SQL> create table t_child2 (child2_id int primary key, parent_id int);Table created.SQL> alter table t_child2 add constraint FK_t_child2 foreign key (parent_id) references t_parent (parent_id) on delete cascade;Table altered.SQL> insert into t_child2 values (2,2);1 row created.SQL> commit;Commit complete.
(3)SET NULL类型
SQL> create table t_child3 (child2_id int primary key, parent_id int);Table created.SQL> alter table t_child3 add constraint FK_t_child3 foreign key (parent_id) references t_parent (parent_id) on delete set null;Table altered.SQL> insert into t_child3 values (3,3);1 row created.SQL> commit;Commit complete.
3.确认主表和子表中的数据
SQL> select * from T_PARENT;PARENT_ID NAME---------- ---------- 1 record1 2 record2 3 record3SQL> select * from T_CHILD1;CHILD1_ID PARENT_ID---------- ---------- 1 1SQL> select * from T_CHILD2; CHILD2_ID PARENT_ID---------- ---------- 2 2SQL> select * from T_CHILD3; CHILD2_ID PARENT_ID---------- ---------- 3 3
4.尝试对具有默认类型外键参照的主表记录进行删除
SQL> delete from T_PARENT where parent_id = 1;delete from T_PARENT where parent_id = 1*ERROR at line 1:ORA-02292: integrity constraint (HBHE.FK_T_CHILD1) violated - child recordfoundSQL> select * from T_CHILD1; CHILD1_ID PARENT_ID---------- ---------- 1 1
在此类型下，不允许删除操作
5.尝试对具有delete cascade类型外键参照的主表记录进行删除
SQL> delete from T_PARENT where parent_id = 2;1 row deleted.SQL> select * from T_CHILD2;no rows selected
级联删除成功
6.尝试对具有delete set null类型外键参照的主表记录进行删除
SQL> delete from T_PARENT where parent_id = 3;1 row deleted.SQL> select * from T_CHILD3; CHILD2_ID PARENT_ID---------- ---------- 3
主表记录可以完成删除，子表中对应的内容被设置为NULL。
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