按多列分组

如何在LINQ中对多个列进行分组

在SQL中与此类似：

SELECT * FROM <TableName> GROUP BY <Column1>,<Column2>

如何将其转换为LINQ：

QuantityBreakdown
(
    MaterialID int,
    ProductID int,
    Quantity float
)

INSERT INTO @QuantityBreakdown (MaterialID, ProductID, Quantity)
SELECT MaterialID, ProductID, SUM(Quantity)
FROM @Transactions
GROUP BY MaterialID, ProductID

使用匿名类型。

例如

group x by new { x.Column1, x.Column2 }

程序样本

.GroupBy(x => new { x.Column1, x.Column2 })

确定为：

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();

对于“按多列分组”，请改用此方法...

GroupBy(x=> new { x.Column1, x.Column2 }, (key, group) => new 
{ 
  Key1 = key.Column1,
  Key2 = key.Column2,
  Result = group.ToList() 
});

用同样的方法可以添加Column3，Column4等。

从C＃7开始，您还可以使用值元组：

group x by (x.Column1, x.Column2)

要么

.GroupBy(x => (x.Column1, x.Column2))

C＃7.1或更大的使用Tuples和Inferred tuple element names（目前它只能linq to objects当需要表达式树例如，它不支持someIQueryable.GroupBy(...)。Github的问题）：

// declarative query syntax
var result = 
    from x in inMemoryTable
    group x by (x.Column1, x.Column2) into g
    select (g.Key.Column1, g.Key.Column2, QuantitySum: g.Sum(x => x.Quantity));

// or method syntax
var result2 = inMemoryTable.GroupBy(x => (x.Column1, x.Column2))
    .Select(g => (g.Key.Column1, g.Key.Column2, QuantitySum: g.Sum(x => x.Quantity)));

C＃3或更高版本，可使用anonymous types：

// declarative query syntax
var result3 = 
    from x in table
    group x by new { x.Column1, x.Column2 } into g
    select new { g.Key.Column1, g.Key.Column2, QuantitySum = g.Sum(x => x.Quantity) };

// or method syntax
var result4 = table.GroupBy(x => new { x.Column1, x.Column2 })
    .Select(g => 
      new { g.Key.Column1, g.Key.Column2 , QuantitySum= g.Sum(x => x.Quantity) });

您也可以将Tuple <>用于强类型分组。

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),
}

尽管此问题询问的是按类分组属性，但是如果要针对ADO对象（如DataTable）按多列分组，则必须将“新”项分配给变量：

EnumerableRowCollection<DataRow> ClientProfiles = CurrentProfiles.AsEnumerable()
                        .Where(x => CheckProfileTypes.Contains(x.Field<object>(ProfileTypeField).ToString()));
// do other stuff, then check for dups...
                    var Dups = ClientProfiles.AsParallel()
                        .GroupBy(x => new { InterfaceID = x.Field<object>(InterfaceField).ToString(), ProfileType = x.Field<object>(ProfileTypeField).ToString() })
                        .Where(z => z.Count() > 1)
                        .Select(z => z);

var Results= query.GroupBy(f => new { /* add members here */  });

.GroupBy(x => x.Column1 + " " + x.Column2)

.GroupBy(x => (x.MaterialID, x.ProductID))

通过新的{x.Col，x.Col}分组x

需要注意的一点是，您需要为Lambda表达式发送一个对象，并且不能为类使用实例。

例：

public class Key
{
    public string Prop1 { get; set; }

    public string Prop2 { get; set; }
}

这将编译，但每个周期将生成一个密钥。

var groupedCycles = cycles.GroupBy(x => new Key
{ 
  Prop1 = x.Column1, 
  Prop2 = x.Column2 
})

如果您不想命名关键属性，然后检索它们，则可以这样做。这将GroupBy正确并为您提供关键属性。

var groupedCycles = cycles.GroupBy(x => new 
{ 
  Prop1 = x.Column1, 
  Prop2= x.Column2 
})

foreach (var groupedCycle in groupedCycles)
{
    var key = new Key();
    key.Prop1 = groupedCycle.Key.Prop1;
    key.Prop2 = groupedCycle.Key.Prop2;
}

对于VB和匿名/ lambda：

query.GroupBy(Function(x) New With {Key x.Field1, Key x.Field2, Key x.FieldN })