Null behaviour on SQL 2008

Question

Sorry guys, is there anyone who can explain me the null behaviour on SQL 2008? I mean, I got this select
select customername, contract, CONVERT(varchar(6),settlementdate,112) as per, sum(consumption) as
weight, SUM(gp2) as gp, SUM(tot) as abp, SUM(ccost) as ccost,  
from big15 group by customername, contract, CONVERT(varchar(6),settlementdate,112)                                                                                                                               
That retrieves me
Warning: Null value is eliminated by an aggregate or other SET operation.
(22118 row(s) affected)
Now I'm wondering, what means? That it has considered the null as zero value? Or that it has truncate or dropped the rows cointaning a null value? It's something I lost in my brain.
And in this case:
select customername, contractid, CONVERT(varchar(6),settlementdate,112) as per,   gp,  abp,  ccost,  RcCost,
(gp + abp + ccost + rccost) as tot                 
That return null if there is a null value. Now
I am going to try Isnull or Coalesce but what concern me is the first question. In a sum column what is the rule on SQL. Zero or drop ( don't consider) the row?

Answer 1

>>>Warning: Null value is eliminated by an aggregate or other SET operation.
You have SUM aggregation function which returns NULL for some data....(you may have COALESCE function to handle the NULLs)
>>>(gp + abp + ccost + rccost) as tot  
Again use (coalesce(gp,0)
+ coalesce(abp,0) + coalesce(ccost,0) + coalesce(rccost,0)) as tot  
Best Regards,Uri Dimant SQL Server MVP,
http://sqlblog.com/blogs/uri_dimant/
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