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