Oracle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。实际体验一下他们对删除操作的不同效果。1.创建主表及子表并简单初始化几条数据1)创建主表t_parent，并初始化三条记录sec@ora10g> create table t_parent (parent_id int primary key, name varchar2(10));sec@ora10g> insert into t_parent values (1,'record1');sec@ora10g> insert into t_parent values (2,'record2');sec@ora10g> insert into t_parent values (3,'record3');sec@ora10g> commit;2)创建三种类型的子表t_child1、t_child2和t_child3(1)no action类型sec@ora10g> create table t_child1 (child1_id int primary key, parent_id int);sec@ora10g> alter table t_child1 add constraint FK_t_child1 foreign key (parent_id) references t_parent (parent_id);sec@ora10g> insert into t_child1 values (1,1);sec@ora10g> commit;(1)cascade类型sec@ora10g> create table t_child2 (child2_id int primary key, parent_id int);sec@ora10g> alter table t_child2 add constraint FK_t_child2 foreign key (parent_id) references t_parent (parent_id) on delete cascade;sec@ora10g> insert into t_child2 values (2,2);sec@ora10g> commit;(1)set null类型sec@ora10g> create table t_child3 (child2_id int primary key, parent_id int);sec@ora10g> alter table t_child3 add constraint FK_t_child3 foreign key (parent_id) references t_parent (parent_id) on delete set null;sec@ora10g> insert into t_child3 values (3,3);sec@ora10g> commit;2.确认主表和子表中的数据sec@ora10g> select * from T_PARENT;PARENT_ID NAME---------- ------------------------------1 record12 record23 record3sec@ora10g> select * from T_CHILD1;CHILD1_ID PARENT_ID---------- ----------1 1sec@ora10g> select * from T_CHILD2;CHILD2_ID PARENT_ID---------- ----------2 2sec@ora10g> select * from T_CHILD3;CHILD2_ID PARENT_ID---------- ----------3 33.尝试对具有默认类型外键参照的主表记录进行删除sec@ora10g> delete from T_PARENT where parent_id = 1;delete from T_PARENT where parent_id = 1*ERROR at line 1:ORA-02292: integrity constraint (SEC.FK_T_CHILD1) violated - child record foundsec@ora10g> select * from T_CHILD1;CHILD1_ID PARENT_ID---------- ----------1 1在此类型下，不允许删除。4.尝试对具有delete cascade类型外键参照的主表记录进行删除sec@ora10g> delete from T_PARENT where parent_id = 2;1 row deleted.sec@ora10g> select * from T_CHILD2;no rows selected成功，级联删除成功。5.尝试对具有delete set null类型外键参照的主表记录进行删除sec@ora10g> delete from T_PARENT where parent_id = 3;1 row deleted.sec@ora10g> 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。6.小结以上就是在Oracle数据库，当主表信息删除后对应的子表中记录的三种不同的处理方式，针对具体的应用场合请选择合适类型。Good luck.secooler10.03.21-- The End --
