【标准SQL的外键约束条件】sql
1): 子表引用父表的主键学习
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在标准SQL上作的扩展】url
1): 只要在父表上有在对应的列上建索引，那么这个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 | +----+----------+------+
【个人评介】.net
主外键约束在标准SQL下体现的是一种一对多的关系，可是通过MySQL的拓展以后能够表现出“多对多”的关系；虽然MySQL这样设计
的设计有必定的灵活性，我的以为最好仍是使用标准SQL的方式。code
【学习交流】blog
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