在创建外键约束时追加on delete cascade或on delete set null
2011-07-25 17:35
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on delete cascade:
外键指向记录被删除时级联删除受影响记录。
删除dept的记录时级联删除emp数据。
on delete set null:
外键指向记录被删除时设置受影响记录外键字段为空。
删除dept的记录时emp中参照这条数据的两条数据的dept的值被自动赋空。
外键指向记录被删除时级联删除受影响记录。
alter table emp add constraint fk_emp_dept foreign key(dept) references dept(deptno) on delete cascade;
删除dept的记录时级联删除emp数据。
on delete set null:
外键指向记录被删除时设置受影响记录外键字段为空。
alter table emp add constraint fk_emp_dept foreign key(dept) references dept(deptno) on delete set null;
删除dept的记录时emp中参照这条数据的两条数据的dept的值被自动赋空。
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