用户-角色多对多查询sql

多对多是常见的对应关系：在开发中 用户和角色就属于对多对多关系：对应查询sql为：

第一步导入数据：

导入user表：

/*
 Navicat Premium Data Transfer

 Source Server         : 本地数据库
 Source Server Type    : MySQL
 Source Server Version : 50726
 Source Host           : localhost:3306
 Source Schema         : laboratory

 Target Server Type    : MySQL
 Target Server Version : 50726
 File Encoding         : 65001

 Date: 12/02/2020 21:06:46
*/

SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;

-- ----------------------------
-- Table structure for user
-- ----------------------------
DROP TABLE IF EXISTS `user`;
CREATE TABLE `user`  (
  `id` varchar(11) CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL,
  `user_name` varchar(255) CHARACTER SET utf8 COLLATE utf8_unicode_ci NULL DEFAULT NULL,
  `password` varchar(255) CHARACTER SET utf8 COLLATE utf8_unicode_ci NULL DEFAULT NULL,
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = MyISAM CHARACTER SET = utf8 COLLATE = utf8_unicode_ci ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of user
-- ----------------------------
INSERT INTO `user` VALUES ('1', 'admin', '123456');
INSERT INTO `user` VALUES ('2', 'alex', '123456');

SET FOREIGN_KEY_CHECKS = 1;

导入角色表：

/*
 Navicat Premium Data Transfer

 Source Server         : 本地数据库
 Source Server Type    : MySQL
 Source Server Version : 50726
 Source Host           : localhost:3306
 Source Schema         : laboratory

 Target Server Type    : MySQL
 Target Server Version : 50726
 File Encoding         : 65001

 Date: 12/02/2020 21:07:11
*/

SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;

-- ----------------------------
-- Table structure for role
-- ----------------------------
DROP TABLE IF EXISTS `role`;
CREATE TABLE `role`  (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `role_name` varchar(20) CHARACTER SET utf8 COLLATE utf8_unicode_ci NULL DEFAULT NULL,
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = MyISAM AUTO_INCREMENT = 3 CHARACTER SET = utf8 COLLATE = utf8_unicode_ci ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of role
-- ----------------------------
INSERT INTO `role` VALUES (1, 'admin');
INSERT INTO `role` VALUES (2, 'user');

SET FOREIGN_KEY_CHECKS = 1;

导入用户-角色表：

/*
 Navicat Premium Data Transfer

 Source Server         : 本地数据库
 Source Server Type    : MySQL
 Source Server Version : 50726
 Source Host           : localhost:3306
 Source Schema         : laboratory

 Target Server Type    : MySQL
 Target Server Version : 50726
 File Encoding         : 65001

 Date: 12/02/2020 21:07:01
*/

SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;

-- ----------------------------
-- Table structure for user_role
-- ----------------------------
DROP TABLE IF EXISTS `user_role`;
CREATE TABLE `user_role`  (
  `user_id` int(11) NOT NULL,
  `role_id` int(11) NULL DEFAULT NULL
) ENGINE = MyISAM CHARACTER SET = utf8 COLLATE = utf8_unicode_ci ROW_FORMAT = Fixed;

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

SET FOREIGN_KEY_CHECKS = 1;

根据用户id查询用户role

SELECT u.id 用户编号,u.user_name 用户名,
r.id 角色编号 ,r.role_name
FROM role r
INNER JOIN user_role ur ON r.id=ur.role_id
INNER JOIN user u ON u.id=ur.user_id
WHERE u.id=1;