mybatis两表连接查时，为什么必须要查主表id才能拿出数据，不然只返回第一条数据，那个大佬知道原理是什么？？

<resultMap type="cn.kgc.tangcco.hsl.pojo.Student1" id="ss">
        <id property="sid" column="sid"/>
        <result property="sage" column="sage" />
        <result property="sname" column="sname"/>
        <result property="gender" column="gender"/>
        <result property="xuefen" column="xuefen"/>
        <result property="pwd" column="pwd"/>
        <result property="team" column="team"/>
        <result property="role" column="role"/>
        <result property="status" column="status"/>

        <association property="score" javaType="cn.kgc.tangcco.hsl.pojo.Score">         
            <result property="bishi" column="bishi"/>
            <result property="jishi" column="jishi"/>           
            <result property="zong" column="zong"/>         
        </association>      
    </resultMap>

<!--不写s.sid时只返回第一条记录-->
<select id="selectall" resultMap="ss">
        select s.sid,s.sname,s.sage,s.team,s.role,ss.zong from students_copy s LEFT join score ss on  s.sid = ss.sid
    </select>