梳理mysql外键约束的知识点。
1、mysql外键约束只对InnoDb引擎有效；
2、创建外键约束如下：
DROP TABLE IF EXISTS t_demo_product;
CREATE TABLE IF NOT EXISTS t_demo_product(
proid int(20),
proname varchar(20),
price int(10),
PRIMARY KEY(proid)
)ENGINE=InnoDB DEFAULT CHARSET=gbk; DROP TABLE IF EXISTS t_demo_operation;
CREATE TABLE IF NOT EXISTS t_demo_operation(
opid int(10),
proid int(20),
opcount int(10),
PRIMARY KEY(opid),
CONSTRAINT `fk_proid_id` FOREIGN KEY (`proid`) REFERENCES `t_demo_product` (`proid`)
)ENGINE=InnoDB DEFAULT CHARSET=gbk;
3、执行插入操作错误：
insert into t_demo_operation(opid, proid, opcount) values
(1,7,2);
//Cannot add or update a child row: a foreign key constraint fails (`test`.`t_demo_operation`, CONSTRAINT `fk_proid_id` FOREIGN KEY (`proid`) REFERENCES `t_demo_product` (`proid`))
4、执行删除操作错误：
delete * from t_demo_product where proid = 2
//Cannot delete or update a parent row: a foreign key constraint fails (`test`.`t_demo_operation`, CONSTRAINT `fk_proid_id` FOREIGN KEY (`proid`) REFERENCES `t_demo_product` (`proid`))