Mybatis一对多查询的两种姿势,你值得拥有(收藏就完事了)

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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

在这里插入图片描述

订单详情表 tlb_order_detail

在这里插入图片描述

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
在这里插入图片描述

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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