-- 建立測試主表. id 是主鍵.
create table test_main (
id int,
value varchar(10),
primary key(id)
);-- 建立測試子表.
create table test_sub (
id int,
main_id int,
value varchar(10),
primary key(id)
);-- 插入測試主表資料.
insert into test_main(id, value) values (1, 'one');
insert into test_main(id, value) values (2, 'two');
-- 插入測試子表資料.
insert into test_sub(id, main_id, value) values (1, 1, 'oneone');
insert into test_sub(id, main_id, value) values (2, 2, 'twotwo');
缺省外鍵約束方式
mysql> alter table test_sub
-> add constraint main_id_cons
-> foreign key (main_id)
-> references test_main(id);
-> //
query ok, 2 rows affected (0.17 sec)
records: 2 duplicates: 0 warnings: 0
mysql> delete from
-> test_main
-> where
-> id = 1;
-> //
error 1451 (23000): cannot delete or update a parent row: a foreign key constrai
nt fails (`test`.`test_sub`, constraint `main_id_cons` foreign key (`main_id`) r
eferences `test_main` (`id`))
mysql使用下面這個語句刪除外來鍵約束
alter table test_sub drop foreign key main_id_cons;
delete cascade 方式
mysql> alter table test_sub
-> add constraint main_id_cons
-> foreign key (main_id)
-> references test_main(id)
-> on delete cascade//
query ok, 2 rows affected (0.16 sec)
records: 2 duplicates: 0 warnings: 0
mysql> delete from
-> test_main
-> where
-> id = 1;
-> //
query ok, 1 row affected (0.02 sec)
mysql> select
-> *
-> from
-> test_sub;
-> //
+----+---------+--------+
| id | main_id | value |
+----+---------+--------+
| 2 | 2 | twotwo |
+----+---------+--------+
1 row in set (0.00 sec)
mysql使用下面這個語句刪除外來鍵約束
alter table test_sub drop foreign key main_id_cons;
update cascade方式
mysql> alter table test_sub
-> add constraint main_id_cons
-> foreign key (main_id)
-> references test_main(id)
-> on update cascade;
-> //
query ok, 1 row affected (0.14 sec)
records: 1 duplicates: 0 warnings: 0
mysql> update test_main set id = 5 where id = 2
-> //
query ok, 1 row affected (0.01 sec)
rows matched: 1 changed: 1 warnings: 0
mysql> select * from test_sub//
+----+---------+--------+
| id | main_id | value |
+----+---------+--------+
| 2 | 5 | twotwo |
+----+---------+--------+
1 row in set (0.00 sec)
mysql使用下面這個語句刪除外來鍵約束
alter table test_sub drop foreign key main_id_cons;
set null方式
mysql> alter table test_sub
-> add constraint main_id_cons
-> foreign key (main_id)
-> references test_main(id)
-> on delete set null;
-> //
query ok, 1 row affected (0.13 sec)
records: 1 duplicates: 0 warnings: 0
mysql> delete from
-> test_main
-> where
-> id = 5;
-> //
query ok, 1 row affected (0.05 sec)
mysql>
mysql> select
-> *
-> from
-> test_sub;
-> //
+----+---------+--------+
| id | main_id | value |
+----+---------+--------+
| 2 | null | twotwo |
+----+---------+--------+
1 row in set (0.00 sec)
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