Linq查询与多个连接

Question

我使用实体框架与我的数据库一起工作，我需要帮助形成一个LINQ查询，这将帮助我获得列LoginId和AccNumber。每个AccNumber只需要一个LoginId，这与LoginId无关。就目前来看，大约有800K的LoginIds，只有大约3000个AccNumber。

因此，最后我需要1个AccNumber与1个LoginId关联。所以我应该有〜3000行和2列。

这里有我需要连接的表：

项目

项目Id，AccNumber，其他无关列

Block_Item

项目Id，块标识，其他不相关的列

Bookversion_Block

BookversionId，块标识，其他不相关的列

Sudent

登录ID，BookversionId，其他无关列

---

理想情况下，我想用复制这个SQL查询LINQ（只是尽量提供尽可能多的信息）

select max(StudentId) as StudentId, BookletVersionId into #ST from Student 
group by BookletVersionId 

select #ST.BookletVersionId, LoginId into #T1 from #ST 
join Student ST on #ST.StudentId = ST.StudentId 

select BookletVersionId, BlockId into #BVB from BookletVersion_Block 
where 
       BookletVersionId in (select BookletVersionId from #ST) 

select #T1.BookletVersionId, #BVB.BlockId, LoginId into #T2 from #T1 
join BookletVersion_Block #BVB on #T1.BookletVersionId = #BVB.BookletVersionId 

select max(BlockId) as BlockId, ItemId into #BI from Block_Item 
where 
       BlockId in (select BlockId from #T2) 
group by ItemId 

select BookletVersionId, #T2.BlockId, ItemId, LoginId into #T3 from #T2 
join #BI on #T2.BlockId = #BI.BlockId 

select max(LoginId) as LoginId, AccessionNumber from #T3 
join Item I on #T3.ItemId = I.ItemId 
group by AccessionNumber order by LoginId 

这是我试过的，但是，我得到的结果不正确，我找回了183,000条记录。很显然，因为我的LINQ查询不正确，这就是我寻求帮助的原因。

var q = (from items in context.Items 
     join context.Block_Item 
     on items.ItemId equals bi.ItemId into bi 
     join context.BookletVersion_Block 
     on bi.Select(x => x.BlockId).FirstOrDefault() equals BVB.BlockId into BVB 
     join context.Students 
     on BVB.Select(x => x.BookletVersionId).FirstOrDefault() equals st.BookletVersionId into st 
      //'VH098334' 
      select new { LoginId = st.Select(x => x.LoginId).FirstOrDefault().ToString(), AccNum = items.AccessionNumber.ToString() }); 

Answer 1

您可以使用子查询与FirstOrDefault沿着你的SELECT语句中，以获得LoginId

var query = 
    from items in context.Items 
    select new 
    { 
     AccNum = items.AccessionNumber, 
     LoginId = (
     from bi in context.Block_Item 
     join bb in context.BookletVersion_Block on bi.BlockId equals bb.BlockId 
     join st in context.Students on bb.BookversionId equals st.BookversionId 
     where items.ItemId == bi.ItemId 
     select st.LoginId 
    ).FirstOrDefault() 
    };