浅谈resultMap的用法及关联结果集映射

<!-- 查询所有的订单数据 -->
    <!-- resultMap:填入配置的resultMap标签的id值 -->
    <select id="queryOrderAll" resultMap="orderResultMap">
        SELECT id, user_id,
        number,
        createtime, note FROM `order`
    </select>

<!-- resultMap最终还是要将结果映射到pojo上，type就是指定映射到哪一个pojo -->
    <!-- id：设置ResultMap的id -->
    <resultMap type="order" id="orderResultMap">
        <!-- 定义主键 ,非常重要。如果是多个字段,则定义多个id -->
        <!-- property：主键在pojo中的属性名 -->
        <!-- column：主键在数据库中的列名 -->
        <id property="id" column="id" />

        <!-- 定义普通属性 -->
        <result property="userId" column="user_id" />
        <result property="number" column="number" />
        <result property="createtime" column="createtime" />
        <result property="note" column="note" />
    </resultMap>

<resultMap type="order" id="orderUserResultMap">
    <id property="id" column="id" />
    <result property="userId" column="user_id" />
    <result property="number" column="number" />
    <result property="createtime" column="createtime" />
    <result property="note" column="note" />

    <!-- association ：配置一对一属性 -->
    <!-- property:order里面的User属性名 -->
        <!-- javaType:属性类型 -->
    <association property="user" javaType="user">
        <!-- id:声明主键，表示user_id是关联查询对象的唯一标识-->
        <id property="id" column="user_id" />
        <result property="username" column="username" />
        <result property="address" column="address" />
    </association>

</resultMap>

<!-- 一对一关联，查询订单，订单内部包含用户属性 -->
<select id="queryOrderUserResultMap" resultMap="orderUserResultMap">
    SELECT
    o.id,
    o.user_id,
    o.number,
    o.createtime,
    o.note,
    u.username,
    u.address
    FROM
    `order` o
    LEFT JOIN `user` u ON o.user_id = u.id
</select>

@Test
public void testQueryOrderUserResultMap() {
    // mybatis和spring整合，整合之后，交给spring管理
    SqlSession sqlSession = this.sqlSessionFactory.openSession();
    // 创建Mapper接口的动态代理对象，整合之后，交给spring管理
    UserMapper userMapper = sqlSession.getMapper(UserMapper.class);

    // 使用userMapper执行根据条件查询用户，结果封装到Order类中
    List<Order> list = userMapper.queryOrderUserResultMap();
    for (Order o : list) {
        System.out.println(o);
    }
    // mybatis和spring整合，整合之后，交给spring管理
    sqlSession.close();
}

<resultMap type="user" id="userOrderResultMap">
    <id property="id" column="id" />
    <result property="username" column="username" />
    <result property="birthday" column="birthday" />
    <result property="sex" column="sex" />
    <result property="address" column="address" />

    <!-- 配置一对多的关系
        property：填写pojo类中集合类类属性的名称
        javaType：填写集合类型的名称 
    -->
    <collection property="orders" javaType="list" ofType="order">
        <!-- 配置主键，是关联Order的唯一标识 -->
        <id property="id" column="oid" />
        <result property="number" column="number" />
        <result property="createtime" column="createtime" />
        <result property="note" column="note" />
    </collection>
</resultMap>

<!-- 一对多关联，查询订单同时查询该用户下的订单 -->
<select id="queryUserOrder" resultMap="userOrderResultMap">
    SELECT
    u.id,
    u.username,
    u.birthday,
    u.sex,
    u.address,
    o.id oid,
    o.number,
    o.createtime,
    o.note
    FROM
    `user` u
    LEFT JOIN `order` o ON u.id = o.user_id
</select>