Mybatis一对多查询的两种姿势（值得收藏）

前言

最近碰到了Mybatis一对多查询的场景，在这里总结对比下常见的两种实现方式。

本文以常见的订单表和订单详情表来举例说明；

数据库表准备

订单表 tbl_order

订单详情表 tlb_order_detail

ps: 一个订单关联多个订单详情，通过order_no订单号关联；

实例演示

方法一：联合查询ResultMap映射

sql直接关联查询，然后结果集通过resultMap的collection映射

例如 查询订单列表，包括订单详情

Order.java 中新增字段orderDetailList，用于存详情列表

public class Order {    private Integer id;    private String orderNo;    private Date orderTime;    private Date payTime;    private String remark;    /**订单详情*/    private List<OrderDetail> orderDetailList;    //省略get、set

OrderMapper.java 新增查询方法

    List<Order> queryOrderList(Map map);

OrderMapper.xml

<resultMap id="BaseResultMap" type="com.chouxiaozi.mybatisdruid.entity.Order" >    <id column="id" property="id" jdbcType="INTEGER" />    <result column="order_no" property="orderNo" jdbcType="VARCHAR" />    <result column="order_time" property="orderTime" jdbcType="TIMESTAMP" />    <result column="pay_time" property="payTime" jdbcType="TIMESTAMP" />    <result column="remark" property="remark" jdbcType="VARCHAR" />    <collection property="orderDetailList" ofType="com.chouxiaozi.mybatisdruid.entity.OrderDetail">      <id column="d_id" property="id" jdbcType="INTEGER" />      <result column="d_order_no" property="orderNo" jdbcType="VARCHAR" />      <result column="good_name" property="goodName" jdbcType="VARCHAR" />      <result column="good_id" property="goodId" jdbcType="INTEGER" />      <result column="good_count" property="goodCount" jdbcType="INTEGER" />    </collection>  </resultMap>  <select id="queryOrderList" resultMap="BaseResultMap">    SELECT      o.*, d.id as d_id,d.order_no as d_order_no,d.good_name,d.good_id,d.good_count    FROM      tbl_order o        LEFT JOIN tbl_order_detail d ON d.order_no = o.order_no    where 1=1    <if test="orderNo != null and orderNo != ''">      and o.order_no = #{orderNo}    </if>    ORDER BY o.order_time desc  </select>

查询结果展示

[ { “id”: 2, “orderNo”: “DD000002”, “orderTime”: “2021-05-09 12:25:57”, “payTime”: “2021-05-09 12:25:59”, “remark”: “2号订单”, “orderDetailList”: [ { “id”: 5, “orderNo”: “DD000002”, “goodName”: “耳机”, “goodId”: 5, “goodCount”: 1 }, { “id”: 4, “orderNo”: “DD000002”, “goodName”: “手机”, “goodId”: 4, “goodCount”: 1 } ] }, { “id”: 1, “orderNo”: “DD000001”, “orderTime”: “2021-05-09 12:25:37”, “payTime”: “2021-05-09 12:25:41”, “remark”: “1号订单”, “orderDetailList”: [ { “id”: 2, “orderNo”: “DD000001”, “goodName”: “饮料”, “goodId”: 2, “goodCount”: 2 }, { “id”: 1, “orderNo”: “DD000001”, “goodName”: “瓜子”, “goodId”: 1, “goodCount”: 1 }, { “id”: 3, “orderNo”: “DD000001”, “goodName”: “矿泉水”, “goodId”: 3, “goodCount”: 2 } ] }]

原理：sql直接关联查询，然后结果集通过resultMap的collection映射，将order_detail表对应的字段映射到orderDetailList字段中。

优点：条件查询方便；无论是订单表还是详情表如果要进行一些条件过滤的话，非常方便，直接写在where中限制就行。不足：因为是先关联查询，后映射；如果需要进行分页查询的话，这种方式就无法满足。主表2条数据，详情表5条数据，关联之后就是10条，无法得主表进行分页；解决方法，就是先给主表套个子查询limit分页后，然后结果集再跟详情表进行关联查询；

方法二：子查询映射

通过resultMap中collection标签的select属性去执行子查询

还以查询订单列表为例

OrderMapper.java

    List<Order> queryOrderList2(Map map);

OrderMapper.xml

<!--主查询的resultMap-->  <resultMap id="BaseResultMap2" type="com.chouxiaozi.mybatisdruid.entity.Order" >    <id column="id" property="id" jdbcType="INTEGER" />    <result column="order_no" property="orderNo" jdbcType="VARCHAR" />    <result column="order_time" property="orderTime" jdbcType="TIMESTAMP" />    <result column="pay_time" property="payTime" jdbcType="TIMESTAMP" />    <result column="remark" property="remark" jdbcType="VARCHAR" />    <!--select子查询， column 传给子查询的参数-->    <collection property="orderDetailList" ofType="com.chouxiaozi.mybatisdruid.entity.OrderDetail"                select="queryDetail" column="order_no">    </collection>  </resultMap>  <!--主查询的sql-->  <select id="queryOrderList2" resultMap="BaseResultMap2">    SELECT    o.*    FROM    tbl_order o    where 1=1    <if test="orderNo != null and orderNo != ''">      and o.order_no = #{orderNo}    </if>    ORDER BY o.order_time desc  </select>  <!--子查询的resultMap-->  <resultMap id="detailResuleMap" type="com.chouxiaozi.mybatisdruid.entity.OrderDetail">    <id column="id" property="id" jdbcType="INTEGER" />    <result column="order_no" property="orderNo" jdbcType="VARCHAR" />    <result column="good_name" property="goodName" jdbcType="VARCHAR" />    <result column="good_id" property="goodId" jdbcType="INTEGER" />    <result column="good_count" property="goodCount" jdbcType="INTEGER" />  </resultMap>  <!--子查询的sql-->  <select id="queryDetail" resultMap="detailResuleMap">    SELECT      *    FROM      `tbl_order_detail` where order_no = #{order_no}  </select>

查询结果同上个例子一样；

原理：通过collection的select方法去调用子查询；所需参数通过column传递；

优点：无论是分页还是普通查询都能满足；主表增加过滤条件也很方便，直接在主查询的sql中增加where条件就行

缺点：子查询不好增加过滤条件；column只能传递主表已有的字段。下面提供解决方式；ps：column传递多个参数 column=“{prop1=col1,prop2=col2}”

例如：实际场景中，详情表有个状态字段，只展示状态正常的详情，需要过滤详情记录。

本例子没有状态字段，就查询订单列表，详情中不展示瓜子，即详情记录中过滤掉good_id = 1的；

在上个例子基础上修改如下：

调用层传参

Map map = new HashMap();    map.put("goodId", 1);    orderMapper.queryOrderList2(map);

orderMapper.xml中增加传参过滤

展示结果如下：详情中已成功过滤掉瓜子；记住，过滤子查询不会影响主表记录；

[ { “id”: 2, “orderNo”: “DD000002”, “orderTime”: “2021-05-09 12:25:57”, “payTime”: “2021-05-09 12:25:59”, “remark”: “2号订单”, “orderDetailList”: [ { “id”: 4, “orderNo”: “DD000002”, “goodName”: “手机”, “goodId”: 4, “goodCount”: 1 }, { “id”: 5, “orderNo”: “DD000002”, “goodName”: “耳机”, “goodId”: 5, “goodCount”: 1 } ] }, { “id”: 1, “orderNo”: “DD000001”, “orderTime”: “2021-05-09 12:25:37”, “payTime”: “2021-05-09 12:25:41”, “remark”: “1号订单”, “orderDetailList”: [ { “id”: 2, “orderNo”: “DD000001”, “goodName”: “饮料”, “goodId”: 2, “goodCount”: 2 }, { “id”: 3, “orderNo”: “DD000001”, “goodName”: “矿泉水”, “goodId”: 3, “goodCount”: 2 } ] }]

总结

方式 联合查询映射 子查询映射 原理 sql查询完成后再通过resultmap映射结果 主表的数据集循环调用子查询 分页 不支持分页查询，主表套子查询也能实现 支持分页 条件过滤 方便条件过滤 传参也能实现，复杂参数例如list不好传递给子查询 ；子查询过滤不影响主表数据

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