MySQL union index usage example_MySQL

This article describes the MySQL joint index with examples. Share it with everyone for your reference, the details are as follows:

Employee table userid
Department table deptid
Employee department table

Conditions: One employee can correspond to multiple departments

Question: How to set up the database so that userid and deptid cannot be added repeatedly?

uuid userid deptid
111
212
311 (This cannot be added)

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');

Note: Create a joint index create index myindex on employee_dept (employeeid,deptid);

Readers who are interested in more MySQL-related content can check out the special topics on this site: "Summary of MySQL Index Operation Skills", "Comprehensive Collection of MySQL Log Operation Skills", "Summary of MySQL Transaction Operation Skills", "Comprehensive Collection of MySQL Stored Procedure Skills", " Summary of MySQL database lock related skills" and "Summary of commonly used MySQL functions"

I hope this article will be helpful to everyone’s MySQL database planning.