LINQ多个连接与一个左连接

Question

我对LINQ很新，我正在努力进行多重JOIN。 所以，这就是我的数据库结构的样子：

现在，我的查询应该如何，如果我有一个特定的成绩，我想选择 {Student.IndexNo，GradeValue.Value}，但如果特定成绩和特定用户没有成绩值，则应返回null（左连接）？

Answer 1

获得LEFT连接的技巧是使用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

故意在2015年为寻找谷歌点击解决方案的新手发布此信息。我设法破解并以编程方式削减编程。

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
};

在内部生成以下SQL代码

{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] 
}