使用LINQ可以转换数据吗？

Question

我想知道是否可以使用LINQ将数据从以下布局中枢起来：

CustID | OrderDate | Qty
1      | 1/1/2008  | 100
2      | 1/2/2008  | 200
1      | 2/2/2008  | 350
2      | 2/28/2008 | 221
1      | 3/12/2008 | 250
2      | 3/15/2008 | 2150

把它改造成类似这样的形式：

CustID  | Jan- 2008 | Feb- 2008 | Mar - 2008 |
1       | 100       | 350       |  250
2       | 200       | 221       | 2150

- Tim Lentine

Answer 1

像这样的东西？

List<CustData> myList = GetCustData();

var query = myList
    .GroupBy(c => c.CustId)
    .Select(g => new {
        CustId = g.Key,
        Jan = g.Where(c => c.OrderDate.Month == 1).Sum(c => c.Qty),
        Feb = g.Where(c => c.OrderDate.Month == 2).Sum(c => c.Qty),
        March = g.Where(c => c.OrderDate.Month == 3).Sum(c => c.Qty)
    });

GroupBy在Linq中与SQL的工作方式不同。在SQL中，您将获得键和聚合（行/列形状）。而在Linq中，您将获得键和任何元素作为键的子元素（分层形状）。要进行透视，您必须将层次结构投影回所选的行/列形式。

- David B

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在应用枢轴之前，列表是否必须是IEnumerable？或者这也可以在EF的IQueryable上完成（而无需将列表实例化到内存中）？ - Rob Vermeulen

@RobVermeulen 我可以把那个查询语句翻译成SQL，所以我期望EF也能够进行翻译。你可以试试吗？ - Amy B

我进行了测试，它有点可行。虽然SQL Profiler显示EF不会将其转换为（快速的）透视查询，而是几个较慢的子查询。 - Rob Vermeulen

感谢您的回答。我想发布一些LinqPad代码，以便人们可以完成这项工作，所以我在下面“回答”了这个问题。但我不知道如何引用这个答案。 - Grant Johnson

我可以问一下，如果列是动态的而不是静态的，比如（12）个月，我的意思是如果情况是变量课程名称，我该怎么办？ - Anyname Donotcare

Answer 2

我使用linq扩展方法回答了类似的问题：

// order s(ource) by OrderDate to have proper column ordering
var r = s.Pivot3(e => e.custID, e => e.OrderDate.ToString("MMM-yyyy")
    , lst => lst.Sum(e => e.Qty));
// order r(esult) by CustID

(+) 一般性实现
(-) 明显比Amy B的慢

有人可以改进我的实现吗？（即对列和行进行排序的方法）

- Sanjaya.Tio

Answer 3

我认为最简洁的方法是使用查找表：

var query =
    from c in myList
    group c by c.CustId into gcs
    let lookup = gcs.ToLookup(y => y.OrderDate.Month, y => y.Qty)
    select new
    {
        CustId = gcs.Key,
        Jan = lookup[1].Sum(),
        Feb = lookup[2].Sum(),
        Mar = lookup[3].Sum(),
    };

- Enigmativity

Answer 4

以下是使用LINQ将数据透视的通用方法：

IEnumerable<CustData> s;
var groupedData = s.ToLookup( 
        k => new ValueKey(
            k.CustID, // 1st dimension
            String.Format("{0}-{1}", k.OrderDate.Month, k.OrderDate.Year // 2nd dimension
        ) ) );
var rowKeys = groupedData.Select(g => (int)g.Key.DimKeys[0]).Distinct().OrderBy(k=>k);
var columnKeys = groupedData.Select(g => (string)g.Key.DimKeys[1]).Distinct().OrderBy(k=>k);
foreach (var row in rowKeys) {
    Console.Write("CustID {0}: ", row);
    foreach (var column in columnKeys) {
        Console.Write("{0:####} ", groupedData[new ValueKey(row,column)].Sum(r=>r.Qty) );
    }
    Console.WriteLine();
}

其中 ValueKey 是一个特殊的类，代表多维键：

public sealed class ValueKey {
    public readonly object[] DimKeys;
    public ValueKey(params object[] dimKeys) {
        DimKeys = dimKeys;
    }
    public override int GetHashCode() {
        if (DimKeys==null) return 0;
        int hashCode = DimKeys.Length;
        for (int i = 0; i < DimKeys.Length; i++) { 
            hashCode ^= DimKeys[i].GetHashCode();
        }
        return hashCode;
    }
    public override bool Equals(object obj) {
        if ( obj==null || !(obj is ValueKey))
            return false;
        var x = DimKeys;
        var y = ((ValueKey)obj).DimKeys;
        if (ReferenceEquals(x,y))
            return true;
        if (x.Length!=y.Length)
            return false;
        for (int i = 0; i < x.Length; i++) {
            if (!x[i].Equals(y[i]))
                return false;
        }
        return true;            
    }
}

这种方法可以用于对N维数据(n>2)进行分组，适用于较小的数据集。对于大型数据集(多达1百万条记录或更多)，或者当无法硬编码枢轴配置时，我编写了一个特殊的PivotData库(它是免费的):

var pvtData = new PivotData(new []{"CustID","OrderDate"}, new SumAggregatorFactory("Qty"));
pvtData.ProcessData(s, (o, f) => {
    var custData = (TT)o;
    switch (f) {
        case "CustID": return custData.CustID;
        case "OrderDate": 
        return String.Format("{0}-{1}", custData.OrderDate.Month, custData.OrderDate.Year);
        case "Qty": return custData.Qty;
    }
    return null;
} );
Console.WriteLine( pvtData[1, "1-2008"].Value );  

- Vitaliy Fedorchenko

Answer 5

// LINQPad Code for Amy B answer
void Main()
{
    List<CustData> myList = GetCustData();
    
    var query = myList
        .GroupBy(c => c.CustId)
        .Select(g => new
        {
            CustId = g.Key,
            Jan = g.Where(c => c.OrderDate.Month == 1).Sum(c => c.Qty),
            Feb = g.Where(c => c.OrderDate.Month == 2).Sum(c => c.Qty),
            March = g.Where(c => c.OrderDate.Month == 3).Sum(c => c.Qty),
            //April = g.Where(c => c.OrderDate.Month == 4).Sum(c => c.Qty),
            //May = g.Where(c => c.OrderDate.Month == 5).Sum(c => c.Qty),
            //June = g.Where(c => c.OrderDate.Month == 6).Sum(c => c.Qty),
            //July = g.Where(c => c.OrderDate.Month == 7).Sum(c => c.Qty),
            //August = g.Where(c => c.OrderDate.Month == 8).Sum(c => c.Qty),
            //September = g.Where(c => c.OrderDate.Month == 9).Sum(c => c.Qty),
            //October = g.Where(c => c.OrderDate.Month == 10).Sum(c => c.Qty),
            //November = g.Where(c => c.OrderDate.Month == 11).Sum(c => c.Qty),
            //December = g.Where(c => c.OrderDate.Month == 12).Sum(c => c.Qty)          
        });
        
    
    query.Dump();
}

/// <summary>
/// --------------------------------
/// CustID  | OrderDate     | Qty
/// --------------------------------
/// 1       | 1 / 1 / 2008  | 100
/// 2       | 1 / 2 / 2008  | 200
/// 1       | 2 / 2 / 2008  | 350
/// 2       | 2 / 28 / 2008 | 221
/// 1       | 3 / 12 / 2008 | 250
/// 2       | 3 / 15 / 2008 | 2150 
/// </ summary>
public List<CustData> GetCustData()
{
    List<CustData> custData = new List<CustData>
    {
        new CustData
        {
            CustId = 1,
            OrderDate = new DateTime(2008, 1, 1),
            Qty = 100
        },

        new CustData
        {
            CustId = 2,
            OrderDate = new DateTime(2008, 1, 2),
            Qty = 200
        },

        new CustData
        {
            CustId = 1,
            OrderDate = new DateTime(2008, 2, 2),
            Qty = 350
        },

        new CustData
        {
            CustId = 2,
            OrderDate = new DateTime(2008, 2, 28),
            Qty = 221
        },

        new CustData
        {
            CustId = 1,
            OrderDate = new DateTime(2008, 3, 12),
            Qty = 250
        },

        new CustData
        {
            CustId = 2,
            OrderDate = new DateTime(2008, 3, 15),
            Qty = 2150
        },      
    };

    return custData;
}

public class CustData
{
    public int CustId;
    public DateTime OrderDate;
    public uint Qty;
}

- Grant Johnson

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谢谢你的回答，Amy B。 - Grant Johnson

Answer 6

这是最有效的方法：

请查看以下方法。不要每个月都遍历客户组。

var query = myList
    .GroupBy(c => c.CustId)
    .Select(g => {
        var results = new CustomerStatistics();
        foreach (var customer in g)
        {
            switch (customer.OrderDate.Month)
            {
                case 1:
                    results.Jan += customer.Qty;
                    break;
                case 2:
                    results.Feb += customer.Qty;
                    break;
                case 3:
                    results.March += customer.Qty;
                    break;
                default:
                    break;
            }
        }
        return  new
        {
            CustId = g.Key,
            results.Jan,
            results.Feb,
            results.March
        };
    });

或者这个：

var query = myList
    .GroupBy(c => c.CustId)
    .Select(g => {
        var results = g.Aggregate(new CustomerStatistics(), (result, customer) => result.Accumulate(customer), customerStatistics => customerStatistics.Compute());
        return  new
        {
            CustId = g.Key,
            results.Jan,
            results.Feb,
            results.March
        };
    });

完整解决方案：

using System;
using System.Collections.Generic;
using System.Linq;

namespace ConsoleApp
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            IEnumerable<CustData> myList = GetCustData().Take(100);

            var query = myList
                .GroupBy(c => c.CustId)
                .Select(g =>
                {
                    CustomerStatistics results = g.Aggregate(new CustomerStatistics(), (result, customer) => result.Accumulate(customer), customerStatistics => customerStatistics.Compute());
                    return new
                    {
                        CustId = g.Key,
                        results.Jan,
                        results.Feb,
                        results.March
                    };
                });
            Console.ReadKey();
        }

        private static IEnumerable<CustData> GetCustData()
        {
            Random random = new Random();
            int custId = 0;
            while (true)
            {
                custId++;
                yield return new CustData { CustId = custId, OrderDate = new DateTime(2018, random.Next(1, 4), 1), Qty = random.Next(1, 50) };
            }
        }

    }
    public class CustData
    {
        public int CustId { get; set; }
        public DateTime OrderDate { get; set; }
        public int Qty { get; set; }
    }
    public class CustomerStatistics
    {
        public int Jan { get; set; }
        public int Feb { get; set; }
        public int March { get; set; }
        internal CustomerStatistics Accumulate(CustData customer)
        {
            switch (customer.OrderDate.Month)
            {
                case 1:
                    Jan += customer.Qty;
                    break;
                case 2:
                    Feb += customer.Qty;
                    break;
                case 3:
                    March += customer.Qty;
                    break;
                default:
                    break;
            }
            return this;
        }
        public CustomerStatistics Compute()
        {
            return this;
        }
    }
}

- Ali Bayat

Answer 7

按月份对数据进行分组，然后将其投影到一个新的数据表中，每个月份都有自己的列。这个新表就是你的数据透视表。

- mattlant

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我无法想象这会如何运作，但我很好奇并请求您包含一些示例代码。 - Josh Gallagher