关联两个表为了把一个字段转码。但是，表中有的空值转码之后这条记录就没有了

怎么能让有无法转码空值的记录也被查出来，空值就不用转码了
SELECT t.codename AS jb, l.dqsprxm, z.codename AS spzt, l.ztmc, l.bmmc,
l.sqrxm, l.fykmmc, l.jkxq, l.bxfx, l.bxcz, l.fyhjj, l.xjhjj, l.ssxmbh,
l.xmmc,l.xmsshymc, l.bz, l.qtsm, l.byo, l.byt,
TO_CHAR (l.ysssny, 'YYYY-MM') ysssny, l.ysbh, l.cjrxm,
TO_CHAR (l.cjsj, 'YYYY-MM-DD HH24:MI:SS') cjsj, l.rid, l.splcfid,
l.xmsshy, l.bxzt, l.fykm, l.jspbmbh, b.ys_rid, l.yssqlb
FROM edubis.neuedu_ys_splxxb b,

   edubis.neuedu_view_ysbmspgl_3 l,
   (SELECT codevalue, codename
      FROM edubis.up_codelist
     WHERE codekind = 'NEUEDU_SPL_SPJB') t,
   (SELECT codevalue, codename
      FROM edubis.up_codelist
     WHERE codekind = 'NEUEDU_GS_SPZT') z

WHERE b.rid = l.splcfid
--AND w.ID = l.xmsshy
AND t.codevalue = b.jb
AND z.codevalue = l.spzt

AND l.yssqlb = 'YGYS'

转码之前先进行判断 ifnull

参考自：
MySQL控制流程函数 http://www.data.5helpyou.com/article327.html