使用mybatis根据主键id获取信息，偶尔查出的数据为NULL，解决根据主键id获取null值，MyBatis-plus的根据id获取信息返回nulll可以参考

业务场景：项目使用了PageHelper和MyBatis,在根据id获取信息时，有时候返回的是null值，报空指针异常。

产生问题的原因

PageHelper默认会在sql 上添加一个Count,会导致执行效率变慢和异常
下图是自己写的SQL

空指针异常后的sql

解决方式：重写PageHelper的Count

示例：

dao层
      AlumniUser findOnlyByAluserId(AlumniUser alumniUser);
//重写count
    long findOnlyByAluserId_COUNT(AlumniUser alumniUser);
xml
    <select id="findOnlyByAluserId" parameterType="mobile.bean.AlumniUser" resultMap="BaseResultMap">
         select aluser.alumniuser_id,aluser.`name`,aluser.mobile_no,aluser.email,aluser.head_img,gar.grade_name as gradeName,ma.major_name as majorName
       from alumniuser aluser
        left join educationinfo educ on educ.alumniuser_id=aluser.alumniuser_id
        left join grade gar on educ.grade_id=gar.grade_id
        left join workinfo woinfo on woinfo.alumniuser_id=aluser.alumniuser_id
        left join major ma on educ.major_id=ma.major_id
    where aluser.alumniuser_id=#{alumniuserId}
    group by  aluser.alumniuser_id
    </select>

    <select id="findOnlyByAluserId_COUNT" parameterType="mobile.bean.AlumniUser" resultType="long">
         select  COUNT(1)
         from alumniuser aluser
        left join educationinfo educ on educ.alumniuser_id=aluser.alumniuser_id
        left join grade gar on educ.grade_id=gar.grade_id
        left join workinfo woinfo on woinfo.alumniuser_id=aluser.alumniuser_id
        left join major ma on educ.major_id=ma.major_id
    where aluser.alumniuser_id=#{alumniuserId}
    group by  aluser.alumniuser_id
    </select>

重写PageHelper的参考链接：https://blog.csdn.net/weixin_43917392/article/details/115511352

标签：alumniuser,获取信息,join,aluser,主键,educ,id,left
来源： https://blog.csdn.net/LOVE320722/article/details/120460122