准备工作，1.创建表

CREATE TABLE IF NOT EXISTS `users` (
    `id` INTEGER NOT NULL AUTO_INCREMENT,
    `name` VARCHAR(255) NOT NULL,
    PRIMARY KEY (`id`)
) ENGINE = InnoDB CHARSET = utf8mb4;

CREATE TABLE IF NOT EXISTS `applications` (
    `id` INTEGER NOT NULL AUTO_INCREMENT,
    `systemName` VARCHAR(255) NOT NULL,
    PRIMARY KEY (`id`)
) ENGINE = InnoDB CHARSET = utf8mb4;

CREATE TABLE IF NOT EXISTS `user_application` (
    `id` INTEGER NOT NULL AUTO_INCREMENT,
    `applicationId` INTEGER NOT NULL,
    `userId` INTEGER NOT NULL,
    `type` TINYINT,
    UNIQUE `user_application_applicationId_userId_unique` (`applicationId`, `userId`),
    PRIMARY KEY (`id`)
) ENGINE = InnoDB CHARSET = utf8mb4;

2.创建数据

INSERT INTO `users` (`name`) VALUES ('张三'),('李四'),('老王'),('小明'),('涛哥');
INSERT INTO `applications` (`systemName`) VALUES ('mac'),('windows'),('linux'),('android'),('qilin');
INSERT INTO `user_application` (`applicationId`,`userId`,`type`) VALUES (1,1,1),(2,1,2),(3,1,2),(2,2,1),(2,2,4);

3.使用多对多 关联查询... 一个用户可以有多个应用，一个应用对应多个用户

查询“张三”下应用：

SELECT users.id, users.name, user_application.type, applications.systemName
FROM users
    LEFT JOIN (user_application `user_application`
        INNER JOIN applications `applications` ON user_application.applicationId = applications.id) ON users.id = user_application.userId
WHERE users.name = '张三'

查询“windows”下用户：

SELECT applications.systemName, users.name, user_application.type
FROM applications
    LEFT JOIN (user_application `user_application`
        INNER JOIN users `users` ON user_application.userId = users.id) ON applications.id = user_application.applicationId
WHERE applications.systemName = 'windows'