Mybatis两表查询——多对多

饼和鱼
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声明

这是我学习了Mybatis后所进行的一些总结笔记，各位若是感兴趣可以去看相关视频，讲得非常不错，废话也不多。

说明

多对多的意思是一个表中的多条记录连接查询时对应另一个表中的多条记录。如下图：

image.png

sql语句为：

        select u.*,r.id as rid, r.role_name, r.role_desc from role r
         left outer join user_role ur on r.id = ur.rid
         left outer join user u on u.id = ur.uid

这样的查询还需要通过一个中间关系表

image.png

数据库中表的创建代码直接放在这里了：

DROP TABLE IF EXISTS `user`;

CREATE TABLE `user` (
  `id` int(11) NOT NULL auto_increment,
  `username` varchar(32) NOT NULL COMMENT '用户名称',
  `birthday` datetime default NULL COMMENT '生日',
  `sex` char(1) default NULL COMMENT '性别',
  `address` varchar(256) default NULL COMMENT '地址',
  PRIMARY KEY  (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

insert  into `user`(`id`,`username`,`birthday`,`sex`,`address`) values (41,'老王','2018-02-27 17:47:08','男','北京'),(42,'小二王','2018-03-02 15:09:37','女','北京金燕龙'),(43,'小二王','2018-03-04 11:34:34','女','北京金燕龙'),(45,'传智播客','2018-03-04 12:04:06','男','北京金燕龙'),(46,'老王','2018-03-07 17:37:26','男','北京'),(48,'小马宝莉','2018-03-08 11:44:00','女','北京修正');


DROP TABLE IF EXISTS `role`;
CREATE TABLE `role` (
  `ID` int(11) NOT NULL COMMENT '编号',
  `ROLE_NAME` varchar(30) default NULL COMMENT '角色名称',
  `ROLE_DESC` varchar(60) default NULL COMMENT '角色描述',
  PRIMARY KEY  (`ID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

insert  into `role`(`ID`,`ROLE_NAME`,`ROLE_DESC`) values (1,'院长','管理整个学院'),(2,'总裁','管理整个公司'),(3,'校长','管理整个学校');


DROP TABLE IF EXISTS `user_role`;

CREATE TABLE `user_role` (
  `UID` int(11) NOT NULL COMMENT '用户编号',
  `RID` int(11) NOT NULL COMMENT '角色编号',
  PRIMARY KEY  (`UID`,`RID`),
  KEY `FK_Reference_10` (`RID`),
  CONSTRAINT `FK_Reference_10` FOREIGN KEY (`RID`) REFERENCES `role` (`ID`),
  CONSTRAINT `FK_Reference_9` FOREIGN KEY (`UID`) REFERENCES `user` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

insert  into `user_role`(`UID`,`RID`) values (41,1),(45,1),(41,2);

1.编写对象实体类

image.png

这里需要两个对象实体类，分别是 User 和 Role

public class Role implements Serializable {

    private Integer roleId;
    private String roleName;
    private String roleDesc;

    //多对多的映射关系；一个角色可以赋予多个用户
    private List<User> users;

    public List<User> getUsers() {
        return users;
    }

    public void setUsers(List<User> users) {
        this.users = users;
    }

    @Override
    public String toString() {
        return "Role{" +
                "roleId=" + roleId +
                ", roleName='" + roleName + '\'' +
                ", roleDesc='" + roleDesc + '\'' +
                '}';
    }

    public Integer getRoleId() {
        return roleId;
    }

    public void setRoleId(Integer roleId) {
        this.roleId = roleId;
    }

    public String getRoleName() {
        return roleName;
    }

    public void setRoleName(String roleName) {
        this.roleName = roleName;
    }

    public String getRoleDesc() {
        return roleDesc;
    }

    public void setRoleDesc(String roleDesc) {
        this.roleDesc = roleDesc;
    }
}

public class User implements Serializable {

    private Integer id;
    private String username;
    private String address;
    private String sex;
    private Date birthday;

    //多对多的映射：一个用户可以具备多个角色
    private List<Role> roles;

    public List<Role> getRoles() {
        return roles;
    }

    public void setRoles(List<Role> roles) {
        this.roles = roles;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", username='" + username + '\'' +
                ", address='" + address + '\'' +
                ", sex='" + sex + '\'' +
                ", birthday=" + birthday +
                '}';
    }



    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

    public String getSex() {
        return sex;
    }

    public void setSex(String sex) {
        this.sex = sex;
    }

    public Date getBirthday() {
        return birthday;
    }

    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }
}

2.编写Role持久层接口

public interface IRoleDao {

    /**
     * 查询所有角色
     * @return
     */
    List<Role> findAll();

}

3.编写映射文件

image.png

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.zhouman.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 property="id" column="id"></id>
            <result property="username" column="username"></result>
            <result property="address" column="address"></result>
            <result property="sex" column="sex"></result>
            <result property="birthday" column="birthday"></result>
        </collection>
    </resultMap>
    
    <!-- 查询所有 -->
    <select id="findAll" resultMap="roleMap">
        select u.*,r.id as rid, r.role_name, r.role_desc from role r
         left outer join user_role ur on r.id = ur.rid
         left outer join user u on u.id = ur.uid
    </select>
</mapper>

4.编写测试方法

image.png

public class RoleTest {

    private InputStream inputStream;
    private SqlSession sqlSession;
    private IRoleDao roleDao;

    @Before
    public void init() throws Exception {
        //1.读取配置文件，生成字节输入流
        inputStream = Resource.class.getResourceAsStream("/SqlMapConfig.xml");
        //2.获取SqlSessionFactory对象
        SqlSessionFactoryBuilder builder = new SqlSessionFactoryBuilder();
        SqlSessionFactory factory = builder.build(inputStream);
        //3.获取SqlSession对象
        sqlSession = factory.openSession();
        //4.通过SqlSession生产dao的代理对象
        roleDao = sqlSession.getMapper(IRoleDao.class);
    }

    @After
    public void destroy() throws Exception {
        //提交事务
        sqlSession.commit();

        inputStream.close();
        sqlSession.close();
    }

    /**
     * 查询所有
     * @throws Exception
     */
    @Test
    public void testFindAll() throws Exception {
        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());
        }
    }

}

另外说明

上面的是 Role 连接 User

那么 User 连接 Role 呢？

image.png

sql语句微调为：

select u.*,r.id as rid, r.role_name, r.role_desc from user u
 left outer join user_role ur on u.id = ur.uid
 left outer join role r on r.id = ur.rid

然后再 User 加上：

    //多对多的映射：一个用户可以具备多个角色
    private List<Role> roles;

    public List<Role> getRoles() {
        return roles;
    }

    public void setRoles(List<Role> roles) {
        this.roles = roles;
    }

再然后就是去编写对应的映射文件：

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.zhouman.dao.IUserDao">

    <!-- 定义User的resultMap -->
    <resultMap id="userMap" type="user">
        <id property="id" column="id"></id>
        <result property="username" column="username"></result>
        <result property="address" column="address"></result>
        <result property="sex" column="sex"></result>
        <result property="birthday" column="birthday"></result>
        <collection property="roles" ofType="role">
            <id property="roleId" column="rid"></id>
            <result property="roleName" column="ROLE_NAME"></result>
            <result property="roleDesc" column="ROLE_DESC"></result>
        </collection>
    </resultMap>

    <!--查询所有，同时获取到用户下所有账户的信息-->
    <select id="findAll" resultMap="userMap">
        select u.*,r.id as rid, r.role_name, r.role_desc from user u
         left outer join user_role ur on u.id = ur.uid
         left outer join role r on r.id = ur.rid
    </select>
</mapper>

最后测试方法

public class UserTest {

    private InputStream inputStream;
    private SqlSession sqlSession;
    private IUserDao userDao;

    @Before
    public void init() throws Exception {
        //1.读取配置文件，生成字节输入流
        inputStream = Resource.class.getResourceAsStream("/SqlMapConfig.xml");
        //2.获取SqlSessionFactory对象
        SqlSessionFactoryBuilder builder = new SqlSessionFactoryBuilder();
        SqlSessionFactory factory = builder.build(inputStream);
        //3.获取SqlSession对象
        sqlSession = factory.openSession();
        //4.通过SqlSession生产dao的代理对象
        userDao = sqlSession.getMapper(IUserDao.class);
    }

    @After
    public void destroy() throws Exception {
        //提交事务
        sqlSession.commit();

        inputStream.close();
        sqlSession.close();
    }

    /**
     * 查询所有
     * @throws Exception
     */
    @Test
    public void testFindAll() throws Exception {
        List<User> users = userDao.findAll();
        for(User user : users){
            System.out.println("-----每个用户的信息------");
            System.out.println(user);
            System.out.println(user.getRoles());
        }
    }
}
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