使用 LINQ 进行数据透视

Question

我遇到了这个问题..我有一个格式如下的 CSV 文件（客户，购买的商品对）：

customer1 item1
customer1 item2
customer1 item3
customer2 item4
customer2 item2
customer3 item5
customer3 item1
customer3 item2
customer4 item1
customer4 item2
customer5 item5
customer5 item1

现在，我希望在查询结果中显示：

item x; item y; how many customers have bought itemx and item together

例如：

item1 item2 3 (because cust1 and cust2 and cust3 bought item1 and item2 together)
item1 item5 1 (because cust5 and cust3 bought item1 and item5 together)

查询返回客户成对购买的所有可能的商品组合。另请注意，Pair(x, y) 与 Pair(y, x) 相同。

SQL 查询如下所示：

SELECT a1.item_id, a2.item_id, COUNT(a1.cust_id) AS how_many_custs_bought_both
  FROM data AS a1
INNER JOIN data AS a2
  ON a2.cust_id=a1.cust_id AND a2.item_id<>a1.item_id AND a1.item_id<a2.item_id
GROUP BY a1.item_id, a2.item_id

您将如何在 C# 中做到这一点 1) 使用常规 for/foreach 循环 2) 使用 LINQ ？

我首先尝试在 LINQ 中执行此操作，但当我注意到 LINQ 不支持 join 子句中的多个 equals 关键字时卡住了。然后我尝试使用普通循环，但是它变得如此低效，以至于它每秒只能处理 30 行（CSV 文件行）。

请指教！

Answer 1

使用 LINQ（并遵循 Tim 回答的前 5 行）将链式方法语法与连接部分的查询语法相结合：

var custItems =  new [] {
    new { customer = 1, item = 1 },
    new { customer = 1, item = 2 },
    new { customer = 1, item = 3 },
    new { customer = 2, item = 4 },
    new { customer = 2, item = 2 },
    new { customer = 3, item = 5 },
    new { customer = 3, item = 1 },
    new { customer = 3, item = 2 },
    new { customer = 4, item = 1 },
    new { customer = 4, item = 2 },
    new { customer = 5, item = 5 },
    new { customer = 5, item = 1 }
};
};

var pairs = custItems.GroupBy(x => x.customer)
         .Where(g => g.Count() > 1)
         .Select(x =>  (from a in x.Select( y => y.item )
                        from b in x.Select( y => y.item )
                        where a < b //If you want to avoid duplicate (a,b)+(b,a)
                        // or just: where a != b, if you want to keep the dupes.
                        select new { a, b}))
         .SelectMany(x => x)
         .GroupBy(x => x)
         .Select(g => new { Pair = g.Key, Count = g.Count() })
         .ToList();

pairs.ForEach(x => Console.WriteLine(x));

编辑：忘记了 OP 想要对出现计数，添加了另一个 .GroupBy() 魔法。

编辑：完成示例以显示它将输出的内容：

{ Pair = { a = 1, b = 2 }, Count = 3 }
{ Pair = { a = 1, b = 3 }, Count = 1 }
{ Pair = { a = 2, b = 3 }, Count = 1 }
{ Pair = { a = 2, b = 4 }, Count = 1 }
{ Pair = { a = 1, b = 5 }, Count = 2 }
{ Pair = { a = 2, b = 5 }, Count = 1 }

编辑：回滚并将字符串更改为整数，因为 OP 显示了一个以整数作为 ID 的数据集，因此不再需要 .GetHashCode()

Answer 2

也许：

var lines = File.ReadLines(csvFilePath);
var custItems = lines
    .Select(l => new { split = l.Split() })
    .Select(x => new { customer = x.split[0].Trim(), item = x.split[1].Trim() })
    .ToList();

var groups = from ci1 in custItems
             join ci2 in custItems
             on ci1.customer equals ci2.customer
             where ci1.item != ci2.item 
             group new { Item1 = ci1.item, Item2 = ci2.item } by new { Item1 = ci1.item, Item2 = ci2.item } into ItemGroup
             select ItemGroup;

var result = groups.Select(g => new
{
    g.Key.Item1,
    g.Key.Item2,
    how_many_custs_bought_both = g.Count()
});

请注意，由于自连接，当文件很大时，ToList 的具体化很重要。

{ Item1 = item1, Item2 = item2, how_many_custs_bought_both = 3 }
{ Item1 = item1, Item2 = item3, how_many_custs_bought_both = 1 }
{ Item1 = item2, Item2 = item1, how_many_custs_bought_both = 3 }
{ Item1 = item2, Item2 = item3, how_many_custs_bought_both = 1 }
{ Item1 = item3, Item2 = item1, how_many_custs_bought_both = 1 }
{ Item1 = item3, Item2 = item2, how_many_custs_bought_both = 1 }
{ Item1 = item4, Item2 = item2, how_many_custs_bought_both = 1 }
{ Item1 = item2, Item2 = item4, how_many_custs_bought_both = 1 }
{ Item1 = item5, Item2 = item1, how_many_custs_bought_both = 2 }
{ Item1 = item5, Item2 = item2, how_many_custs_bought_both = 1 }
{ Item1 = item1, Item2 = item5, how_many_custs_bought_both = 2 }
{ Item1 = item2, Item2 = item5, how_many_custs_bought_both = 1 }

Answer 3

你可以这样写：

IDictionary<int, int> pivotResult = customerItems.ToLookup(c => c.Customer)
                                 .ToDictionary(x=>x.Key, y=>y.Count());

Answer 4

工作 LINQ 示例，不太漂亮！

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

class Data
{
    public Data(int cust, int item)
    {
        item_id = item;
        cust_id = cust;
    }
    public int item_id { get; set; }
    public int cust_id { get; set; }

    static void Main(string[] args)
    {
        var data = new List<Data>
                       {new Data(1,1),new Data(1,2),new Data(1,3),
                        new Data(2,4),new Data(2,2),new Data(3,5),
                        new Data(3,1),new Data(3,2),new Data(4,1),
                        new Data(4,2),new Data(5,5),new Data(5,1)};

           (from a1 in data
            from a2 in data
            where a2.cust_id == a1.cust_id && a2.item_id != a1.item_id && a1.item_id < a2.item_id
            group new {a1, a2} by new {item1 = a1.item_id, item2 = a2.item_id}
            into g
            select new {g.Key.item1, g.Key.item2, count = g.Count()})
            .ToList()
            .ForEach(x=>Console.WriteLine("{0} {1} {2}",x.item1,x.item2,x.count))
            ;
           Console.Read();
    }
}

输出：

1 2 3
1 3 1
2 3 1
2 4 1
1 5 2
2 5 1