LINQ查詢表達式中的子查詢

Question

下面是我的SQL查詢：

select e.id, m.id, e.Name, (select 1 where e.Id Is Null) as NotExists
    from    Employee e 
            INNER JOIN Department d
            ON e.Id = d.Id  
            LEFT JOIN Manager m
            ON e.Id = m.Id
            order by e.Name

我試圖像上面這樣將上面的查詢寫到LINQ中：

var result = from e in employeeRep
             join d in departmentRep
             on e.Id equals d.Id
             join m in manager
             on e.Id = m.Id
             order by e.Name
             select new
             {
               Name = e.Name,
               EmpId = e.Id,
               ManagerId = m.Id,
               DepartmentId = d.Id,
               DeparmentName = d.Name,
               NotExists = // here I want to write this query (select 1 where e.Id Is Null) as NotExists
             } ;

誰能告訴我如何實現這一目標

謝謝

Answer 1

如果Id nullable為nullable則可以執行以下操作：

select new
             {
               Name = e.Name,
               EmpId = e.Id,
               ManagerId = m.Id,
               DepartmentId = d.Id,
               DeparmentName = d.Name,
               NotExists = e.Id == null ? 1 : 0 

            }

Answer 2

您可以使用let關鍵字。 它允許您創建一個子查詢，為其分配一個名稱，然后在以后的代碼中使用它們。 雖然，它在許多復雜的子查詢中更有用，在這里也可以使用。

var result = from e in employeeRep
         join d in departmentRep
         on e.Id equals d.Id
         join m in manager
         on e.Id = m.Id
         order by e.Name
         let mySub = // write your query here.
         select new
         {
           Name = e.Name,
           EmpId = e.Id,
           ManagerId = m.Id,
           DepartmentId = d.Id,
           DeparmentName = d.Name,
           NotExists = mySub
         };