Msql 联合索引

员工表 userid

部门表 deptid

员工部门表

条件：一个员工可以对应多个部门

问题：怎么样设置数据库，让其不能重复添加 userid 和deptid？

uuid userid deptid

111

212

311(这个就不能让其添加)

DROP TABLE IF EXISTS `dept`;
CREATE TABLE `dept` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `deptname` char(32) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of dept
-- ----------------------------
INSERT INTO `dept` VALUES ('1', '1');
INSERT INTO `dept` VALUES ('2', '2');

DROP TABLE IF EXISTS `employee`;
CREATE TABLE `employee` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(32) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of employee
-- ----------------------------
INSERT INTO `employee` VALUES ('1', '11');

DROP TABLE IF EXISTS `employee_dept`;
CREATE TABLE `employee_dept` (
  `id` int(11) NOT NULL,
  `employeeid` int(11) NOT NULL,
  `deptid` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `bb` (`deptid`),
  KEY `myindex` (`employeeid`,`deptid`),
  CONSTRAINT `aa` FOREIGN KEY (`employeeid`) REFERENCES `employee` (`id`),
  CONSTRAINT `bb` FOREIGN KEY (`deptid`) REFERENCES `dept` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of employee_dept
-- ----------------------------
INSERT INTO `employee_dept` VALUES ('1', '1', '1');
INSERT INTO `employee_dept` VALUES ('2', '1', '2');

备注：创建联合索引create index myindex on employee_dept (employeeid,deptid);

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