SQL question

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