oracle sql别名

为sql的字段起一个别名，常以为是可有可无的，但是有时候是必要的。

例如在ibatis中

 <!-- 获取已发或待发送的彩信记录列表 -->
     <resultMap id="mmsListByRecordTypeMap" class="richinfo.mms.bean.req.MmsRecordReq" >
         <result column="groupid" property="groupId" />
         <result column="subject" property="title" />
         <result column="sendnumber" property="userNumber" />
         <result column="destnumbers" property="destNumber" />
         <result column="istime" property="isTimming" typeHandler="richinfo.mms.util.MyBatisTypeHandlerCallback"/>
         <result column="statuss" property="status" />
         <result column="startsendtime" property="sendTime" />
         <result column="totalsize" property="totalSize" />
     </resultMap>
     <select id="getMmsListByRecordType" resultMap="mmsListByRecordTypeMap"
         resultClass="map">
         select * from (select b.*,rownum r from ( select
         uin,FN_GetDecryptUserNumber(sendnumber) sendnumber,subject,to_char(startsendtime, 'yyyy-mm-dd hh24:mi:ss')
         startsendtime,istime,showfileid,totalsize,showtype,groupid,
         wmsys.wm_concat(FN_GetDecryptUserNumber(destnumber)) destnumbers,wmsys.wm_concat(nvl(status,
         0)) statuss from mms_send_his_record where 1=1
         <isEqual prepend="and" property="recordType" compareValue="0">
             issave=1
         </isEqual>
         <isEqual prepend="and" property="recordType" compareValue="1">
         <![CDATA[
             (istime=0 or (istime=1 and startsendtime<=sysdate)) and issave=1
         ]]>
         </isEqual>
         <isEqual prepend="and" property="recordType" compareValue="2">
         <![CDATA[
             istime=1 and startsendtime>sysdate
             ]]>
         </isEqual>
         and uin = #uin#
         <include refid="condition" />
         <![CDATA[
          group by groupid,uin,FN_GetDecryptUserNumber(sendnumber),subject,startsendtime,istime,showfileid,totalsize,showtype order by startsendtime desc) b
           where rownum <= #endRecord#
          ]]>
         <isNotEmpty property="destNumber">
         <![CDATA[
             and instr(destnumbers,#destNumber#)>0  or instr(destnumbers,fn_encrypt_function(#destNumber#))>0
             ]]>
         </isNotEmpty>
         ) where r > #startRecord#
     </select>

1、必须为第15、17行起别名

　　如果不为第15、17行起别名的话，执行sql语句就会出问题。因为这个select的查询结果集映射在了mmsListByRecordTypeMap上，这时候

如果不起别名，FN_GetDecryptUserNumber(sendnumber)和sendnumber是不相等的，结果就不能映射在userNumber上。

所以select查询的列名和resultMap的column上的名字是必须相等的。

2、group by 语句不需要别名