SQL doubt

Below is my SQL. I have to do a small modification,

I have to execute this 'YEUXFILE.PQTY-COALESCE(YODIAC.PQTY,0) ' only when 
YEUXFILE.PQTY is positive value. (ie. when YEUXFILE.PQTY is greater than or 
equal to zero, I have to do that substraction)

Can some one help me to implement this in my already existing solution. I tried 
but could not succeed.

Thanks for your help, I sincerely appreciate your help.

 

 

 

 

SELECT YEUXFILE.PRECID,YEUXFILE.CFTC,                    

      YEUXFILE.PEXCH,YEUXFILE.PFC,YEUXFILE.ACCOUNT,              

      YEUXFILE.PFIRM,YEUXFILE.PSNAME,YEUXFILE.PQTY-COALESCE(YODIAC.PQTY,0)      

      AS PQTY                                                                   

      FROM QS36F/YEUXFILE                                                       

      LEFT OUTER JOIN (SELECT                                                   

      PRECID,ACCOUNT,PFIRM,PFC,SUM(COALESCE(YODIAC.PQTY,           

      0)) AS                                                                    

       PQTY                                                                     

      FROM  QS36F/YODIAC GROUP BY PRECID,ACCOUNT,PFIRM,PFC)        

      AS                                                                        

      YODIAC                                                                    

      ON YEUXFILE.ACCOUNT = YODIAC.ACCOUNT