SELECT u.*, r.id AS rid, r.role_name, r.role_descFROM role rLEFT OUTER JOIN user_role ur ON r.id = ur.ridLEFT OUTER JOIN USER u ON u.id = ur.uid;

3.编写角色实体类

@Data
public class Role implements Serializable {private Integer roleId;private String roleName;private String roleDesc;//多对多的关系映射：一个角色可以赋予多个用户private List<User> users;}

4.编写 Role 持久层接口

public interface IRoleDao {/*** 查询所有角色* @return*/List<Role> findAll();
}

5.编写映射文件

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="cn.oesoft.mybatis01.dao.IRoleDao"><!--定义 role 表的 ResultMap--><resultMap id="roleMap" type="role"><id property="roleId" column="rid"></id><result property="roleName" column="role_name"></result><result property="roleDesc" column="role_desc"></result><collection property="users" ofType="user"><id column="id" property="id"></id><result column="username" property="username"></result><result column="address" property="address"></result><result column="sex" property="sex"></result><result column="birthday" property="birthday"></result></collection></resultMap><!--查询所有--><select id="findAll" resultMap="roleMap">select u.*, r.id as rid, r.role_name, r.role_descfrom role rleft outer join user_role ur on r.id = ur.ridleft outer join user u on u.id = ur.uid</select>
</mapper>

6.编写测试类

public class RoleTest {private InputStream in;private SqlSession sqlSession;private IRoleDao roleDao;@Before//用于在测试方法执行之前执行public void init()throws Exception{//1.读取配置文件，生成字节输入流in = Resources.getResourceAsStream("SqlMapConfig.xml");//2.获取 SqlSessionFactorySqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in);//3.获取 SqlSession 对象sqlSession = factory.openSession(true);//4.获取 dao 的代理对象roleDao = sqlSession.getMapper(IRoleDao.class);}@After//用于在测试方法执行之后执行public void destroy()throws Exception{//提交事务// sqlSession.commit();//6.释放资源sqlSession.close();in.close();}/*** 测试查询所有*/@Testpublic void testFindAll(){List<Role> roles = roleDao.findAll();for(Role role : roles){System.out.println("---每个角色的信息----");System.out.println(role);System.out.println(role.getUsers());}}
}

二、实现 User 到 Role 的多对多

SELECT u.*, r.id AS rid, r.role_name, r.role_descFROM `USER` uLEFT OUTER JOIN user_role ur ON u.id = ur.uidLEFT OUTER JOIN role r ON r.id = ur.rid;

private List<Role> roles;

/*** 查询所有用户，同时获取出每个用户被分配的所有角色* @return*/List<User> findAllWithRoles();

<!-- 建立 User 实体（包含角色）和数据库表的对应关系type 属性：指定实体类的全限定类名id 属性：给定一个唯一标识，是给查询 select 标签引用用的。--><resultMap type="user" id="userWithRolesMap"><id column="id" property="id"></id><result column="username" property="username"/><result column="address" property="address"/><result column="sex" property="sex"/><result column="birthday" property="birthday"/><!-- collection 是用于建立一对多中集合属性的对应关系ofType 用于指定集合元素的数据类型--><collection property="roles" ofType="role"><id column="rid" property="roleId"/><result column="role_name" property="roleName"/><result column="role_desc" property="roleDesc"/></collection></resultMap><!-- 配置查询所有操作 --><select id="findAllWithRoles" resultMap="userWithRolesMap">SELECT u.*, r.id AS rid, r.role_name, r.role_descFROM `USER` uLEFT OUTER JOIN user_role ur ON u.id = ur.uidLEFT OUTER JOIN role r ON r.id = ur.rid</select>

/*** 测试查询所有用户，包含角色*/@Testpublic void testFindAllWithRoles(){List<User> users = userDao.findAllWithRoles();for(User user:users){System.out.println("---每个用户的信息----");System.out.println(user);System.out.println(user.getRoles());}}