1. 创建表时设置外键
KEY `键名`(`添加外键的字段`),
CONSTRAINT `键名` FOREIGN KEY (`添加外键的字段`) REFERENCES `被引用的表`(`被引用的字段`)
2. 创建表时没外键,建表成功后通过SQL语句添加外键
ALTER TABLE `表名` ADD CONSTRAINT `约束名` FOREIGN KEY(`作为外键的字段`) REFERENCES `被引用的表`(`被引用的字段`);
CREATE TABLE IF NOT EXISTS `grade` (
`id` int NOT NULL AUTO_INCREMENT COMMENT '年级id',
`name` varchar(50) NOT NULL COMMENT '年级名称',
PRIMARY KEY (`id`)
) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_bin COMMENT = '年级表';
DROP TABLE IF EXISTS grade;
CREATE TABLE IF NOT EXISTS`student` (
`id` int NOT NULL AUTO_INCREMENT COMMENT '学号',
`name` varchar(30) NOT NULL DEFAULT '匿名' COMMENT '姓名',
`sex` varchar(2) NOT NULL DEFAULT '隐藏' COMMENT '性别',
`pwd` varchar(20) NOT NULL DEFAULT '123456' COMMENT '密码',
`birthday` datetime NULL DEFAULT NULL COMMENT '出生日期',
`address` varchar(100) NULL DEFAULT NULL COMMENT '家庭住址',
`email` varchar(50) NULL DEFAULT NULL COMMENT '邮箱',
`gradeid` int NOT NULL COMMENT '学生的年级',
PRIMARY KEY (`id`),
KEY `FK_gradeid1`(`gradeid`),
CONSTRAINT `FK_gradeid1` FOREIGN KEY (`gradeid`) REFERENCES `grade`(`id`)
) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_bin COMMENT = '学生表';
DROP TABLE IF EXISTS student;
CREATE TABLE IF NOT EXISTS `grade` (
`id` int NOT NULL AUTO_INCREMENT COMMENT '年级id',
`name` varchar(50) NOT NULL COMMENT '年级名称',
PRIMARY KEY (`id`)
) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_bin COMMENT = '年级表';
DROP TABLE IF EXISTS grade;
CREATE TABLE IF NOT EXISTS`student` (
`id` int NOT NULL AUTO_INCREMENT COMMENT '学号',
`name` varchar(30) NOT NULL DEFAULT '匿名' COMMENT '姓名',
`sex` varchar(2) NOT NULL DEFAULT '隐藏' COMMENT '性别',
`pwd` varchar(20) NOT NULL DEFAULT '123456' COMMENT '密码',
`birthday` datetime NULL DEFAULT NULL COMMENT '出生日期',
`address` varchar(100) NULL DEFAULT NULL COMMENT '家庭住址',
`email` varchar(50) NULL DEFAULT NULL COMMENT '邮箱',
`gradeid` int NOT NULL COMMENT '学生的年级',
PRIMARY KEY (`id`)
) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_bin COMMENT = '学生表';
DROP TABLE IF EXISTS student;
ALTER TABLE `student`
ADD CONSTRAINT `FK_gradeid2` FOREIGN KEY(`gradeid`) REFERENCES `grade`(`id`);