LINQ按多列分组(Group By)并计算总和(Sum) （转载）

来源：https://codedefault.com/2018/group-by-multiple-columns-and-sum-in-csharp

.NET[C#]LINQ按多列分组(Group By)并计算总和(Sum)

SQL语句：

SELECT * FROM <TableName> GROUP BY <Column1>,<Column2>
QuantityBreakdown
(
    MaterialID int,
    ProductID int,
    Quantity float
)

INSERT INTO @QuantityBreakdown (MaterialID, ProductID, Quantity)
SELECT MaterialID, ProductID, SUM(Quantity)
FROM @Transactions
GROUP BY MaterialID, ProductID
在C#中，如何使用LINQ实现以上SQL语句中的需求呢？方式归纳如下：

方式一
var query = (from t in Transactions
             group t by new {t.MaterialID, t.ProductID}
             into grp
                    select new
                    {
                        grp.Key.MaterialID,
                        grp.Key.ProductID,
                        Quantity = grp.Sum(t => t.Quantity)
                    }).ToList();
方式二
from grouping in list.GroupBy(x => new Tuple<string,string,string>(x.Person.LastName,x.Person.FirstName,x.Person.MiddleName))
select new SummaryItem
{
    LastName = grouping.Key.Item1,
    FirstName = grouping.Key.Item2,
    MiddleName = grouping.Key.Item3,
    DayCount = grouping.Count(), 
    AmountBilled = grouping.Sum(x => x.Rate),
}
方式三
var Results= query.GroupBy(x => new { x.Column1, x.Column2 });

易错提醒：

grp.Key.ProductID, //前往不要忘了Key！！！
Quantity = grp.Sum(t => t.Quantity) //必须取别名Quantity ！！！