需求

最近遇到一个事情，前端某个请求只需要后端实体类中的某几个属性，但是数据库中却定义了其他属性，我们如何把其他属性的值排除在外，只获取我们需要的属性值呢？
原本数据库字段，有很多，但是我们只需要某些字段

<resultMap type="SheepGenealogy" id="SheepGenealogyResult">
        <result property="id"    column="id"    />
        <result property="eartagid"    column="eartagid"    />
        <result property="sex"    column="sex"    />
        <result property="farmloc"    column="farmloc"    />
        <result property="bornloc"    column="bornloc"    />
        <result property="region"    column="region"    />
        <result property="buildingnum"    column="buildingnum"    />
        <result property="columnnum"    column="columnnum"    />
        <result property="borntime"    column="borntime"    />
        <result property="bornweight"    column="bornweight"    />
        <result property="breed"    column="breed"    />
        <result property="coatcolor"    column="coatcolor"    />
        <result property="motherid"    column="motherid"    />
        <result property="fatherid"    column="fatherid"    />
        <result property="grandfatherid"    column="grandfatherid"    />
        <result property="grandmotherid"    column="grandmotherid"    />
        <result property="grandpaid"    column="grandpaid"    />
        <result property="grandmaid"    column="grandmaid"    />
        <result property="video"    column="video"    />
        <result property="bak1"    column="bak1"    />
        <result property="bak2"    column="bak2"    />
        <result property="bak3"    column="bak3"    />
        <result property="createBy"    column="create_by"    />
        <result property="createDate"    column="create_date"    />
        <result property="updateBy"    column="update_by"    />
        <result property="updateDate"    column="update_date"    />
        <result property="remarks"    column="remarks"    />
        <result property="delFlag"    column="del_flag"    />
    </resultMap>

解决

自己定义一个resultMap
type=“java.util.HashMap”

   <resultMap type="java.util.HashMap" id="SheepSpeciesResult">
       <result property="breed"    column="breed"    />
       <result property="bak1"    column="bak1"    />
   </resultMap>

sql语句

    <select id="selectSheepGenealogysheepSpecies"  resultMap="SheepSpeciesResult">
        SELECT breed,COUNT(breed) bak1 from sheep_genealogy GROUP BY breed
    </select>

同时把你的service和mapper文件里面的对应的方法返回值类型改成List<Map<String, Object>>
mapper

public List<Map<String, Object>> selectSheepGenealogysheepSpecies();

service

public List<Map<String, Object>> selectSheepGenealogysheepSpecies() {
    System.out.println(sheepGenealogyMapper.selectSheepGenealogysheepSpecies());
    return sheepGenealogyMapper.selectSheepGenealogysheepSpecies();
}