【標準SQL的外鍵約束條件】sql
1): 子表引用父表的主鍵學習
drop table if exists child,parent; create table if not exists parent( id int not null auto_increment primary key, v int ); create table if not exists child( id int not null auto_increment primary key, parent_id int not null, v int, constraint fk__child__parent_id foreign key (parent_id) references parent(id) ); insert into parent(id,v) values(1,100); insert into child(parent_id,v) values(1,1000); insert into child(parent_id,v) values(1,1000); select * from parent; +----+------+ | id | v | +----+------+ | 1 | 100 | +----+------+ select * from child; +----+-----------+------+ | id | parent_id | v | +----+-----------+------+ | 1 | 1 | 1000 | | 2 | 1 | 1000 | +----+-----------+------+
2): 子表引用交表的惟一索引ui
create table if not exists parent( id int not null, v int, constraint unique index uix__parent_id (id) ); create table if not exists child( id int not null auto_increment primary key, parent_id int not null, v int, constraint fk__child__parent_id foreign key (parent_id) references parent(id) ); insert into parent(id,v) values(1,100); insert into child(parent_id,v) values(1,1000); insert into child(parent_id,v) values(1,1000); select * from parent; +----+------+ | id | v | +----+------+ | 1 | 100 | +----+------+ select * from child; +----+-----------+------+ | id | parent_id | v | +----+-----------+------+ | 1 | 1 | 1000 | | 2 | 1 | 1000 | +----+-----------+------+
【innodb在標準SQL上作的擴展】url
1): 只要在父表上有在對應的列上建索引，那麼這個列就能在子表中引用spa
create table if not exists parent( id int not null auto_increment primary key, v int, index uix__parent_v (v) -- 只要父表上有索引就行 ); create table if not exists child( id int not null auto_increment primary key, parent_v int not null, v int, constraint fk__child__parent_v foreign key (parent_v) references parent(v) -- 在子表中引用 ); insert into parent(id,v) values(1,100); insert into parent(id,v) values(2,100); insert into child(parent_v,v) values(100,2000); insert into child(parent_v,v) values(100,2000); select * from parent; +----+------+ | id | v | +----+------+ | 1 | 100 | | 2 | 100 | +----+------+ select * from child; +----+----------+------+ | id | parent_v | v | +----+----------+------+ | 1 | 100 | 2000 | | 2 | 100 | 2000 | +----+----------+------+
【個人評介】.net
主外鍵約束在標準SQL下體現的是一種一對多的關係，可是通過MySQL的拓展以後能夠表現出「多對多」的關係；雖然MySQL這樣設計
的設計有必定的靈活性，我的以爲最好仍是使用標準SQL的方式。code
【學習交流】blog
-----------------------------http://www.sqlpy.com-------------------------------------------------索引
-----------------------------http://www.sqlpy.com-------------------------------------------------