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 record3 SQL> select * from T_CHILD1; CHILD1_ID PARENT_ID ---------- ---------- 1 1 SQL> select * from T_CHILD2; CHILD2_ID PARENT_ID ---------- ---------- 2 2 SQL> 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 record found SQL> 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。