BTW, I have tried to convert the query into a left like this

select BLPCID,sum(BLGROSS_SALES),sum(BLRETURNS),sum(BLNETSALES),
sum(BLDELIVERY_QUANTITY),sum(BLBACKLOG)
from BACKLOG left join TURNOVER_PC_CS on BLPCID=TO6PCID
join CUSTOMERS on TO6CSID=CSID and CSUSID1=56 and CSGPID3=65
where TO6PCID is null
group by BLPCID

but I don't get the right results. Any idea, where the error could be?

Regards

Guido