Question

我会尽量简而言之：我是LINQ的新手。在那里。

我有三个表，已将EntityFramework集成到我的解决方案中：

Employees        Desks          Rooms
--------------   ------------   ------------
FirstName        DeskId         RoomId
DeskId           RoomId         RoomName
                 Color          HasWindows
                 Width

我需要查看：

Employee   Room name              
----------------------------------
Lumberg    Corner Office         
Milton     Storage in basement   
Peter      Cubicle 214

因此，我创建了一个EmployeeRoomListViewModel，并在LINQ中使用JOINs进行了填充。除无汗外我缺少一些记录。即那些没有分配办公桌的员工（DeskId为空）。我也需要那些。

这是我的LINQ：

from Emp in db.Employees 
join D in db.Desks on D.DeskId equals Emp.DeksId
join R in db.Rooms on D.RoomId equals R.RoomId
          select new EmployeeRoomListViewModel
          {
              Employee = Emp.FirstName,
              Room = R.RoomName
          }).ToList();

有什么想法吗？

Answer 1

您的查询是正确的，但您只需要在其中添加“ Into Alias”和DefaultIfEmpty（）。

from Emp in db.Employees 
join Des in db.Desks on Des.DeskId equals Emp.DeksId into SomeThing 
from Des in SomeThing.DefaultIfEmpty();

Answer 2

您需要对左外部联接使用DefaultIfEmpty子句

from Emp in db.Employees 
  join D in db.Desks on D.DeskId equals Emp.DeksId into employeeDesks 
    from ed in employeeDesks.DefaultIfEmpty() 
  join R in db.Rooms on D.RoomId equals R.RoomId into deskRooms
    from dr in deskRooms.DefaultIfEmpty()
  select new EmployeeRoomListViewModel
  {
      Employee = Emp.FirstName,
      Room = R.RoomName
  }).ToList();

Answer 3

这将解决您的问题

from Emp in db.Employees 
join D in db.Desks on D.DeskId equals Emp.DeksId into desklist from deskjoin in 
desklist.DefaultIfEmpty()
join R in db.Rooms on deskjoin.RoomId equals R.RoomId into roomlist from roomjoin in roomlist.DefaultIfEmpty() select new EmployeeRoomListViewModel
                      {
                          Employee = Emp.FirstName,
                          Room = roomjoin.RoomName
                      }).ToList();

第一次需要将左连接雇员表和Desks表一起使用，然后需要左雇员的雇员和Desk表联接的结果

在此“从desklist中的deskjoin中进入桌面列表中。DefaultIfEmpty（）”用于左联接查询

缺少“空”记录

3 个答案: