LINQ與一個左連接的多個連接

Question

我對LINQ相當陌生，而且我正努力做出一個多JOIN。 所以，這是我的數據庫結構是怎樣的樣子：

現在，應該如何查詢我的樣子，如果我有一個特定的等級，我想選擇 {Student.IndexNo，GradeValue.Value }，但是如果對於特定等級和特定用戶沒有等級值，則應該返回null（左連接）？

Answer 1

的手段來得到一個LEFT JOIN是使用DefaultIfEmpty（）方法：

var otherValue = 5; 

var deps = from tbl1 in Table1 
      join tbl2 in Table2 
       on tbl1.Key equals tbl2.Key into joinGroup 
      from j in joinGroup.DefaultIfEmpty() 
      where 
       j.SomeProperty == "Some Value" 
       && tbl1.OtherProperty == otherValue 
      select j; 

Answer 2

故意張貼這在2015年的新手找對谷歌打的解決方案。我設法破解和編程我的方式進入解決方案。

var projDetails = from r in entities.ProjekRumah 
join d in entities.StateDistricts on r.ProjekLocationID equals d.DistrictID 
join j in entities.ProjekJenis on r.ProjekTypeID equals j.TypeID 
join s in entities.ProjekStatus on r.ProjekStatusID equals s.StatusID 
join approvalDetails in entities.ProjekApproval on r.ProjekID equals approvalDetails.ProjekID into approvalDetailsGroup 
from a in approvalDetailsGroup.DefaultIfEmpty() 
select new ProjectDetailsDTO() 
     { 
     ProjekID = r.ProjekID, 
     ProjekName = r.ProjekName, 
     ProjekDistrictName = d.DistrictName, 
     ProjekTypeName = j.TypeName, 
     ProjekStatusName = s.StatusName, 
     IsApprovalAccepted = a.IsApprovalAccepted ? "Approved" : "Draft", 
     ProjekApprovalRemarks = a.ApprovalRemarks 
}; 

主要生產以下SQL代碼內部

{SELECT [Extent1].[ProjekID] AS [ProjekID] 
    ,[Extent1].[ProjekName] AS [ProjekName] 
    ,[Extent2].[DistrictName] AS [DistrictName] 
    ,[Extent3].[TypeName] AS [TypeName] 
    ,[Extent4].[StatusName] AS [StatusName] 
    ,CASE 
     WHEN ([Extent5].[IsApprovalAccepted] = 1) 
      THEN N'Approved' 
     ELSE N'Draft' 
     END AS [C1] 
    ,[Extent5].[ApprovalRemarks] AS [ApprovalRemarks] 
FROM [dbo].[ProjekRumah] AS [Extent1] 
INNER JOIN [dbo].[StateDistricts] AS [Extent2] ON [Extent1].[ProjekLocationID] = [Extent2].[DistrictID] 
INNER JOIN [dbo].[ProjekJenis] AS [Extent3] ON [Extent1].[ProjekTypeID] = [Extent3].[TypeID] 
INNER JOIN [dbo].[ProjekStatus] AS [Extent4] ON [Extent1].[ProjekStatusID] = [Extent4].[StatusID] 
LEFT JOIN [dbo].[ProjekApproval] AS [Extent5] ON [Extent1].[ProjekID] = [Extent5].[ProjekID] 
}