MySQL中外键的创建、约束以及删除

alter table 表名 add constraint 约束名 foreign key(当前表中约束的字段) references 主表表名(要约束的字段名);
alter table student add constraint fk_class_student foreign key(cls_id) class(cls_id) on update cascade on delete no action;

CREATE TABLE student(
sid int PRIMARY KEY,
cls_id int not null,
sname varchar(10) not null,
constraint fk_class_student foreign key(cls_id) references class(cls_id) on update cascade on delete no action
)ENGINE=InnoDB DEFAULT CHARSET=utf8;

SHOW CREATE TABLE class

CREATE TABLE `class` (
 `cls_id` int NOT NULL,
 `cls_name` varchar(15) NOT NULL,
 PRIMARY KEY (`cls_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

delete from class WHERE cls_id=1

delete from student WHERE cls_id=1

UPDATE class set cls_id=4 where cls_id=1

alter table 子表名　drop foreign key 外键约束名
alter table student drop foreign key fk_class_student