[大数据]Mybatis之联表查询(多对多)

使用 <resultMap>标签以及<association>和<collection>子标签，进行关联查询.

Pojo里面的User类

public class User implements Serializable {
    private Integer id;
    private String username;
    private String address;
    private Date birthday;
    private String sex;

    private List<Role> roles;

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

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

    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 Date getBirthday() {
        return birthday;
    }

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

    public String getSex() {
        return sex;
    }

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

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

Pojo里面的Role类

public class Role implements Serializable {
    private Integer roleId;
    private String roleName;
    private String roleDesc;
    private List<User> users;

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

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

    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;
    }

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

Dao层的接口

 List<Role> findAll();//查询所有

IRoleDao.xml

<?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.xia.dao.IRoleDao">
    <!--别名只是给类取的,他这个resultMap是因为类里面的变量名与数据库字段名不一样-->
    <!--定义Role表的resultMap-->
    <resultMap id="roleMap" type="com.xia.domo.Role">
        <id property="roleId" column="rid"></id>
        <result column="role_name" property="roleName"></result>
        <result column="role_desc" property="roleDesc"></result>
        <collection property="users" ofType="com.xia.domo.User" >
            <id property="id" column="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>
    <!--查询所有角色-->
    <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 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>

user表:

user_role表:

role表:

此处sql查询的结果为:

查询如有不懂参考:SQL 内连接（inner join）与外连接（left outer join 、right outer join ）区别.

测试代码:

public class RoleTest {

    private InputStream in;
    private SqlSession sqlSession;
    private IRoleDao iRoleDao;
    @Before//用于在测试方法执行之前执行
    public void init() throws IOException {
        //1.读取配置文件，形成字节输入流
        in = Resources.getResourceAsStream("SqlMapConfig.xml");
        //2.获取SqlSessionFactory对象
        SqlSessionFactory factory= new SqlSessionFactoryBuilder().build(in);
        //3.SqlSession对象
        sqlSession=factory.openSession();
        //4.获取dao的代理对象
        iRoleDao = sqlSession.getMapper(IRoleDao.class);
        //5.执行查询所有方法
    }

    @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();
    }

    /**
     * 测试查询所有角色，同时获取用户信息
     * @throws IOException
     */
    @Test
    public void TestFindAll() throws IOException {

        List<Role> roles = iRoleDao.findAll();
        for (Role role:roles) {
            System.out.println(role);
            System.out.println(role.getUsers());
        }

    }
}

结果:

结果相符,很nice!