Mysql 外键约束

参考： http://blog.sina.com.cn/s/blog_53729e4601011wja.html

http://blog.163.com/sejin@126/blog/static/827504552010101544755364/

http://blog.csdn.net/xubo578/article/details/8203550 E-R图的

MySQL中定义外键的表,双方必须都是Innodb。

子表中外键字段和其对应父表中的字段必须都设为索引，主键自动为索引

//E-R图中几种关系
Identifying Relationship:需要提供外键给另外一张表作联合主键  
Non-identifying Mandatory Relationship:强制外键  
Non-identifying Optional Relationship:可选外键  
One-to-one Relationship   Non-specific：无特殊关系    


判别一个关系是Non-Identifying还是Identifying只要区分子实体的主键，看是否需要父实体的外键来共同作为主键(个人理解类似联合主键？)，需要则为Identifying，如果子实体自己的主键就可唯一标识则它为Non-Identifying！   //貌似Mysql Workbench中 虚线1:1关联是添加外键的，实线是添加外键为主键的

 ALTER TABLE yourtablename
    ADD [CONSTRAINT 外键名] FOREIGN KEY [id] (index_col_name, ...)    //外键名 不能加引号，单双引号都不行
    REFERENCES tbl_name (index_col_name, ...)
    [ON DELETE {CASCADE | SET NULL | NO ACTION | RESTRICT}]      // ON DELETE CASCADE   删除父表对应字段时，子表对应外键也会被删除，不能子表操作父表
    [ON UPDATE {CASCADE | SET NULL | NO ACTION | RESTRICT}]      // ON UPDATE CASCADE   修改父表对应字段时，子表对应外键也会更新

//多个外键 个人理解 要把该字段设置为索引，再foreign key
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)
                        REFERENCES product(category, id)
                        ON UPDATE CASCADE ON DELETE RESTRICT,
                      -- 单外键
                      INDEX (customer_id),
                      FOREIGN KEY (customer_id)
                       REFERENCES customer(id)) TYPE=INNODB;

//先给父表，子表中涉及到外键的字段设置索引
ALTER TABLE parts ADD INDEX idx_model (model);
ALTER TABLE pc ADD INDEX idx_cpumodel (cpumodel);

//在给pc表设置外键
ALTER TABLE pc ADD CONSTRAINT fk_cpu_model 
FOREIGN KEY (cpumodel) 
REFERENCES parts(model) ON UPDATE CASCADE;

//删除外键
alter table child drop foreign key 外键名

ps:

1.若不声明on update/delete,则默认是采用restrict方式. //restrict 方式个人理解表示受限，比如ON UPDATE RESTRICT 时，父表对应字段更新，子表中外键不随之更新，同样对应删除
2.对于外键约束,最好是采用: ON UPDATE CASCADE ON DELETE RESTRICT 的方式.