mysql中的外键操作 建表添加外键 删除外键约束 追加外键约束 0523

关键字

foreign key

外键演练

新建老师生表，演练外键的相关操作

主表创建

# 建库
CREATE DATABASE dbSchool0523 CHARACTER SET utf8

# 用库
USE dbSchool0523

# 建主表
CREATE TABLE teacher(
  id INT PRIMARY KEY AUTO_INCREMENT,
  NAME VARCHAR(5) NOT NULL
)

# 主表添加数据
INSERT INTO teacher VALUES(NULL,'自来也');

# 查看主表数据
SELECT * FROM teacher;

创建子表同时定义外键

单词

建子表代码

# 新建子表学生表 建表时定义外键
CREATE TABLE student(
  id INT PRIMARY KEY AUTO_INCREMENT COMMENT '编号',
  NAME VARCHAR(10) NOT NULL COMMENT '学生姓名',
  tid INT COMMENT '老师id',
  CONSTRAINT fk_student_teacher FOREIGN KEY(tid) REFERENCES teacher(id)
);

# 查看状态
DESC student;

核心代码

CONSTRAINT fk_student_teacher FOREIGN KEY(tid) REFERENCES teacher(id)

删除外键约束

删前查看建表语句

SHOW CREATE TABLE student;

CREATE TABLE `student` (
  `id` int(11) NOT NULL auto_increment COMMENT '编号',
  `name` varchar(10) NOT NULL COMMENT '学生姓名',
  `tid` int(11) default NULL COMMENT '老师id',
  PRIMARY KEY  (`id`),
  KEY `fk_student_teacher` (`tid`),
  CONSTRAINT `fk_student_teacher` FOREIGN KEY (`tid`) REFERENCES `teacher` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

删除外键

ALTER TABLE student DROP FOREIGN KEY fk_student_teacher;

删除后查看建表语句

SHOW CREATE TABLE student;

CREATE TABLE `student` (
  `id` int(11) NOT NULL auto_increment COMMENT '编号',
  `name` varchar(10) NOT NULL COMMENT '学生姓名',
  `tid` int(11) default NULL COMMENT '老师id',
  PRIMARY KEY  (`id`),
  KEY `fk_student_teacher` (`tid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

给无外键的表追加外键

代码

ALTER TABLE student ADD CONSTRAINT fk2_student_teacher FOREIGN KEY (tid) REFERENCES teacher(id);

结构图

查看建表语句

SHOW CREATE TABLE student;

CREATE TABLE `student` (
  `id` int(11) NOT NULL auto_increment COMMENT '编号',
  `name` varchar(10) NOT NULL COMMENT '学生姓名',
  `tid` int(11) default NULL COMMENT '老师id',
  PRIMARY KEY  (`id`),
  KEY `fk2_student_teacher` (`tid`),
  CONSTRAINT `fk2_student_teacher` FOREIGN KEY (`tid`) REFERENCES `teacher` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8