MyBatis关联查询（二、一对多查询）

MyBatis关联查询（二、一对多查询）

需求：查询所有用户信息及用户关联的账户信息。

分析：用户信息和他的账户信息为一对多关系，并且查询过程中如果用户没有账户信息，此时也要将用户信息查询出来，此时左外连接查询比较合适。
基础项目搭建参考第一个mybatis项目的创建和读取数据库信息
MyBatis关联查询（一、一对一查询）

在上面项目的基础上先编写user表的pojo文件

public class User implements Serializable {
    private Integer id;
    private String username;
    private Date birthday;
    private String sex;
    private String address;
    //多方
    private List<Account> accountList;

    public Integer getId() {
        return id;
    }

    public String getUsername() {
        return username;
    }

    public List<Account> getAccountList() {
        return accountList;
    }

    public void setAccountList(List<Account> accountList) {
        this.accountList = accountList;
    }

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

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

    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;
    }

    public String getAddress() {
        return address;
    }

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

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

编写UserMapper.java代码

public interface UserMapper {
    User getUserById(Integer id);

}

在UserMapper.xml中编写下列代码

<resultMap id="getUserByIdResult" type="cn.fpl1116.pojo.User">
        <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"/>

        <!--一堆多使用collection标签指定数据的封装规则-->
        <collection property="accountList" ofType="cn.fpl1116.pojo.Account">
            <id column="aid" property="id"></id>
            <result column="uid" property="uid"></result>
            <result column="money" property="money"></result>
        </collection>
    </resultMap>

    <select id="getUserById" parameterType="int" resultMap="getUserByIdResult">
        SELECT u.*, a.id aid, a.uid uid, a.money money FROM user u LEFT JOIN account a ON u.id=a.uid WHERE u.id=#{id}
    </select>

编写测试方法

@Test
    public void testGetUserById(){
        UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
        User user = userMapper.getUserById(41);
        System.out.println(user);
    }

运行结果如下：