LINQ - LEFT JOIN

Question

这是我的旧代码：

var destinctList = HoldingsList.DistinctBy(x => new { x.ClientName, x.ProductName }).Select(x => new { ClientName = x.ClientName, ClientProductName = x.ProductName }).ToList(); 

foreach (var item in destinctList) 
{ 

    var clientAlias = (from ca in someContext.ClientAlias 
         where ca.Name == item.ClientName 
         orderby ca.CreationDate descending 
         select ca).FirstOrDefault(); 

    if (clientAlias != null) 
    { 
     destinctList.Where(x => x.ClientName == item.ClientName).ForEach(x => x.ClientID = clientAlias.ClientID); 
    } 

    ClientProductAlias clientProductAlias = null; 

    if (clientAlias != null) 
    { 
     clientProductAlias = (from ca in someContext.ClientProductAlias 
           where ca.Name == item.ClientProductName 
           && ca.ClientID == clientAlias.ClientID 
           orderby ca.CreationDate descending 
           select ca).FirstOrDefault(); 
    } 

    if (clientProductAlias != null) 
    { 
     destinctList.Where(x => x.ClientProductName == item.ClientProductName).ForEach(x => x.ClientProductID = clientProductAlias.ClientProductID); 
    } 
} 

这完美地工作，但它采取长。

现在我想使所有的工作与联接。这是我到目前为止：

(from list in destinctList 
join ca in someContext.ClientAlias on list.ClientName equals ca.Name into list_client_join 
from list_client in list_client_join.DefaultIfEmpty 
join cpa in someContext.ClientProductAlias on new { ClientID = (long)list.ClientID, Name = list.ClientProductName } equals new { cpa.ClientID, cpa.Name } into j1 
from j2 in j1.DefaultIfEmpty 
orderby list_client.CreationDate descending 
orderby j2.CreationDate descending 
select new { ClientID = list_client.ClientID, 
      ClientName = list.ClientName, 
      ClientProductID = j2.ClientProductID, 
      ClientProductName = list.ClientProductName }).ToList(); 

这是我到目前为止，但没有返回任何东西。我究竟做错了什么？

Answer 1

我注意到你的DefaultIfEmpty语句缺少括号，而且我认为问题在于它如何处理连接到空数据。我认为你肯定需要拨打where j1.Any()，你可能还需要where list_client_join.Any()。

(from list in destinctList 
join ca in someContext.ClientAlias on list.ClientName equals ca.Name into list_client_join 
//where list_client_join.Any() 
from list_client in list_client_join.DefaultIfEmpty() 
join cpa in someContext.ClientProductAlias on new { ClientID = (long)list.ClientID, Name = list.ClientProductName } equals 
    new { cpa.ClientID, cpa.Name } into j1 
// maybe needs the following: 
where j1.Any() 
from j2 in j1.DefaultIfEmpty() 
orderby list_client.CreationDate descending 
orderby j2.CreationDate descending 
select new { ClientID = list_client.ClientID, 
      ClientName = list.ClientName, 
      ClientProductID = j2.ClientProductID, 
      ClientProductName = list.ClientProductName }).ToList(); 

更新：

// alternate query 
(from list in destinctList 
let ca = someContext.ClientAlias 
      .OrderByDescending (cca => cca.CreationDate) 
      .FirstOrDefault (cca => cca.Name == list.ClientName) 
let cca = someContext.ClientProductAlias 
      .OrderByDescending (ccpa => ccpa.CreationDate) 
      .FirstOrDefault(ccpa => int.Equals(ccpa.ClientID, 
          ca == null ? -1 : ca.ClientID) && 
          string.Equals(ccpa.Name,list.ClientProductName)) 
select new 
{ 
    ClientID = ca != null ? ca.ClientID : -1, 
    ClientName = list.ClientName, 
    ClientProductID = cca != null ? cca.ClientProductID : -1, 
    ClientProductName = list.ClientProductName 
} 
).ToList();  

模拟了使用数据：http://ideone.com/XEqf4