c# - LINQ to Entities への SQL 変換

Question

この SQL ステートメントを LINQ to Entities に変換するにはどうすればよいですか。テーブルを WHERE 句で結合する方法が見つかりません

SQL:

Declare @DID int, @key varchar(50)

SET @DID = 3
SET @key = ''

SELECT TBL.GID, TBL.GName, TBL.DID FROM     
(
  SELECT TOP 1 (@DID*10000) AS [GID], '' AS [GName], @DID AS [DID] 
  FROM dbo.Employees E
  WHERE E.DepartmentID=@DID 
  AND (E.GroupID IS NULL OR E.GroupID = 0)  
  UNION ALL  
  SELECT G.GroupID AS [GID], G.GroupName AS [GName], G.DepartmentID AS [DID] 
  FROM dbo.Groups G
) TBL 
WHERE TBL.DID IN (@DID) 
AND TBL.GName IN 
(
  SELECT (CASE WHEN E.GroupID = 0 or E.GroupID IS NULL THEN '' 
          ELSE (SELECT G.GroupName FROM Groups G WHERE G.GroupID=E.GroupID) END) 
  FROM Employees E 
  WHERE E.DepartmentID = @DID 
  AND (E.FirstName + ' ' + E.LastName) LIKE '%' + @key + '%'
) 
ORDER BY TBL.GName DESC

---

エンティティへの LINQ:

var Details = (from a in Context.Employees
               where a.DepartmentID == DepartmentID
               && (a.GroupID == null || a.GroupID == 0)

               select new
               {
                 GID = Convert.ToInt32(DepartmentID * 10000),
                 GName = "",
                 DID = a.DepartmentID
               }

               ).Concat(
                 from a in Context.Groups
                 select new
                 {
                   GID = a.GroupID,
                   GName = a.GroupName,
                   DID = DepartmentID
                 }
               );