MySQL外键设置详解

;                  FOREIGN KEY (parent_id) REFERENCES parent(id)
                     ON DELETE CASCADE
) TYPE=INNODB;
向parent插入数据后,向child插入数据,插入时,child中的parent_id的值只能是parent中有的数据,否则插入不成功;
删除parent记录时,child中的相应记录也会被删除;-->因为: on delete cascade
更新parent记录时,不给更新;-->因为没定义,默认采用restrict.
4.2
若child如下:
mysql>
create table child(id int not null primary key auto_increment,parent_id int,
index par_ind (parent_id),
constraint fk_1 foreign key (parent_id) references
parent(id) on update cascade on delete restrict)
type=innodb;
用上面的:
1). 则可以更新parent记录时,child中的相应记录也会被更新;-->因为: on update cascade
2). 不能是子表操作,影响父表.只能是父表影响子表.
3). 删除外键:
alter table child drop foreign key fk_1;
添加外键:
alter table child add constraint fk_1 foreign key (parent_id) references
parent(id) on update restrict on delete set null;
(5) 多个外键存在:
product_order表对其它两个表有外键。
一个外键引用一个product表中的双列索引。另一个引用在customer表中的单行索引：
CREATE TABLE product (category INT NOT NULL, id INT NOT NULL,
                      price DECIMAL,
                      PRIMARY KEY(category, id)) TYPE=INNODB;
CREATE TABLE customer (id INT NOT NULL,
                      PRIMARY KEY (id)) TYPE=INNODB;

CREATE TABLE product_order (no INT NOT NULL AUTO_INCREMENT,
                      product_category INT NOT NULL,
                      product_id INT NOT NULL,
                      customer_id INT NOT NULL,
                      PRIMARY KEY(no),
                      -- 双外键
                      INDEX (product_category, product_id),
                      FOREIGN KEY (product_category, product_id)
             &nbs