Question

我是Linq的新手，想知道如何获取客户ID列表及其交易次数

public class Transaction
{
    public int TransactionId {get; set;}
    public int CustomerId {get; set;}   
}


public class Customer
{
    public int ID {get; set;}
    public string Name {get; set;}
    public string Surname {get; set;}
}

我认为我需要通过交易加入客户，但不太确定如何计算。

    var query  =    (from c  in customers
                    join t in transactions on c.ID equals t.CustomerId

Answer 1

var query = transactions
                .GroupBy(t => t.CustomerId)
                    .Select (t => new { Id = t.Key, TranCount = t.Count() })
                        .ToList();

无需加入您拥有有关Transaction对象的所有信息。

如果您需要额外的客户信息（例如客户姓氏），您需要加入，在这种情况下，您可以执行以下操作;

var query  =    (from c  in customers
                join t in transactions on c.ID equals t.CustomerId
                group c by c.ID  into grp 
                select new 
                { 
                    Id = grp.Key, 
                    Surname = grp.First().Surname, 
                    TranCount = grp.Count() 
                }).ToList();

Answer 2

只有每个客户都有交易时，saj的回答才有效。相反，最好使用以Customer开头的群组联接，并计算结果：

var query = from customer in customers
            join transaction in transactions
              on customer.Id equals transaction.CustomerId
              into customerTransactions
            select new { customer.Id, Count = customerTransactions.Count() };

linq加入和统计

2 个答案: