MyBatis|Java8 自连接查询子分类

在这里插入图片描述
类似商品分类、部门树相关构建在开发中必不可少，既可以通过mybatis，也可以通过java8的新特性来实现。

此时只需要蛋糕和点心，然后三级分类为一个List
构造一个VO

public class CategoryVO {
    
    

    private Integer id;
    private String name;
    private String type;
    private Integer fatherId;

    // 三级分类vo list
    private List<SubCategoryVO> subCatList;
    }

可以直接通过collection 标签实现在sql层面将其封装成VO

<resultMap id="myCategoryVO" type="com.imooc.pojo.vo.CategoryVO">
    <id column="id" property="id"/>
    <result column="name" property="name"/>
    <result column="type" property="type"/>
    <result column="fatherId" property="fatherId"/>

    <!--
      collection 标签：用于定义关联的list集合类型的封装规则
      property：对应三级分类的list属性名
      ofType：集合的类型，三级分类的vo
    -->
    <collection property="subCatList" ofType="com.imooc.pojo.vo.SubCategoryVO">
      <id column="subId" property="subId"/>
      <result column="subName" property="subName"/>
      <result column="subType" property="subType"/>
      <result column="subFatherId" property="subFatherId"/>
    </collection>
  </resultMap>

  <select id="getSubCatList" resultMap="myCategoryVO" parameterType="int">
    SELECT
        f.id as id,
        f.`name` as `name`,
        f.type as type,
        f.father_id as fatherId,
        c.id as subId,
        c.`name` as subName,
        c.type as subType,
        c.father_id as subFatherId
    FROM
        category f
    LEFT JOIN
        category c
    on
        f.id = c.father_id
    WHERE
        f.father_id = #{
    
    rootCatId}
  </select>

如下为java8实现，直接使用分组即可，变成键值对的形式

  List<TargetCustomerProductDO> targetCustomerProductDOS = targetCustomerProductService.selectCustomerProductByTargetId(targetId);
    Map<String,List<TargetCustomerProductDO>> map= targetCustomerProductDOS.stream()
            .collect(Collectors.groupingBy(TargetCustomerProductDO::getCompanyName));

根据不用需求可以选用不同方式

MyBatis|Java8 自连接查询子分类

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