CASE WHEN sum(G_SAMAINQTY) is null THEN 0 ELSE sum(G_SAMAINQTY)END

 

select G_SRCORGID,CASE  WHEN sum(G_SAMAINQTY) is null THEN 0 ELSE sum(G_SAMAINQTY)END from(SELECT YLS.T_STDINOUTWHSK.G_SRCORGID, YLS.T_STDINOUTWHSKS.G_SAMAINQTY
  FROM YLS.T_STDINOUTWHSKS
       INNER JOIN YLS.T_STDINOUTWHSK
          ON YLS.T_STDINOUTWHSKS.G_MID = YLS.T_STDINOUTWHSK.G_ID
 WHERE     to_char (YLS.T_STDINOUTWHSK.G_BILLCRTDATE, 'yyyy') = '2018'
       AND YLS.T_STDINOUTWHSK.G_BILLSTATUSCODE = '01')
       group by G_SRCORGID