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var query = (from o in order.Where(x=>x.oid==100)
join p in product
on o.pid equals p.ID into t
from p in t.DefaultIfEmpty()
select p.W).Sum();
(from o in order
join p in product
on o.pid equals p.ID
into temp
from temp.DefaultIfEmpty
where o.oid==100
select temp.W).Sum(x=>x.W);