分组多字段求和
query.GroupBy(q => new { q.Year, q.Month })
.Select(q => new
{
Year = q.Key.Year,
Month = q.Key.Month,
BuildAmount = q.Sum(i => i.BuildAmount),
RecAmount = q.Sum(i => i.RecAmount),
Amount = q.Sum(i => i.Amount),
RealAmount = q.Sum(i => i.RealAmount)
});
不分组多字段求和（这样得到的就是对应字段的总的求和，其实还是利用了分组，不过给分组依据传个空，如果利用linq的话就是传个常数）
where.GroupBy(x => new { }).Select(q => new
{
sumWeight = q.Sum(x => x.Weight),
sumQuantity = q.Sum(x => x.Quantity),
sumIncome = q.Sum(x => x.Income)
}).FirstOrDefault();