mybatis编写mapper.xml时的一些疑惑

mybatis编写mapper.xml时的一些疑惑

1. 今天在写一个需求时，在写sql的时候遇到了极大的问题。一直在报错，
   修改后的代码如下：

 <select id="getEnterCommunity" parameterType="FydjCommper" resultType="FydjCommunityActivities">
        select
        f.fydjca_Actname as fydjcaActname,f.fydjca_Actstime as fydjcaActstime,f.fydjca_Actetime as fydjcaActetime,f.fydjca_Ecname as fydjcaEcname
        from
        fydj_Community_Activities f
        left join
        FYDJ_SINGNMANAGE t
        on
        t.fydjsm_emuuid = f.fydjca_uuid
        where 1 = 1
        <if test="fydjcaUuid!=null and fydjcaUuid!=''">
            and t.fydjsm_useruuid=#{fydjcaUuid,jdbcType=VARCHAR}
        </if>
    </select>

2. 我犯了第一个错是，我的fydjcaUuid是另一个表中的数据，我在parameterType直接填写了与resultType相同的参数FydjCommunityActivities实体类，但是从页面上传递过来的参数是FydjCommper表中的字段，所以要换成FydjCommper。
3. 第二个错误是我直接在select片段下面这样书写了：
   f.fydjca_Actname,f.fydjca_Actstime,f.fydjca_Actetime ,f.fydjca_Ecname
   并没有根据实体类中的参数赋值，当然会报错，切记，下次不可犯同样的错误。
4. 刚好我sql编写的能力较差，尝试用where条件进行查询，和左连接的结果一样，纯属练习，代码如下：

 SELECT *
     FROM (select f.fydjca_Actname  as fydjcaActname,
                  f.fydjca_Actstime as fydjcaActstime,
                  f.fydjca_Actetime as fydjcaActetime,
                  f.fydjca_Ecname   as fydjcaEcname
     from fydj_Community_Activities f, FYDJ_SINGNMANAGE t
     where 1 = 1
            and t.fydjsm_emuuid = f.fydjca_uuid
     <if test="fydjcaUuid!=null and fydjcaUuid!=''">
            and t.fydjsm_useruuid=#{fydjcaUuid,jdbcType=VARCHAR}
     </if>