c# - linq 加入和计数

Question

我是 Linq 的新手，想知道如何获取客户 ID 列表以及他们的交易计数

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;}
}

我想我需要与客户一起进行交易，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();

无需加入您就拥有 Transaction 对象的所有信息。

但是，如果您需要额外的客户信息（例如客户姓氏），则需要加入，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

saj 的回答只有在每个客户都有交易时才有效。相反，最好使用以 开头的组加入Customer，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() };