LINQ有条件的左外连接

Question

我具有来自带有实体框架5的SQL数据库的下表。

[我想做的是选择所有用户，其中tblUserBusiness.BUID等于传入的值，或者其中Users.IsSysAdmin等于True。如果Users.IsSysAdmin等于True，那么将没有相关的tblUserBusiness记录，因此将是“左外部联接”。

<< img src =“ https://image.soinside.com/eyJ1cmwiOiAiaHR0cHM6Ly9pLnN0YWNrLmltZ3VyLmNvbS9VdW9ZWS5wbmcifQ==” alt =“在此处输入图像描述”>

我从下面的LINQ查询开始，该查询已正确过滤，但不允许外部联接

        businessUsers = (From u In db.Users
                            From bu In db.tblUserBusinesses
                            Where bu.BUID.Equals(buID) Or u.IsSysAdmin.Equals(True)
                                Select New Users With {.ID = u.ID,
                                                       .Name = u.Name,
                                                       .UserName = u.UserName}).ToList

然后我转到下面的查询，该查询允许进行外部联接，但我不知道如何实现Where bu.BUID.Equals(buID) Or u.IsSysAdmin.Equals(True)

        businessUsers = (From u In db.Users
                            Group Join bu In db.tblUserBusinesses
                                On u Equals bu.User
                                Into userList = Group
                         Select New Users With {.ID = u.ID,
                                                       .Name = u.Name,
                                                       .UserName = u.UserName}).ToList

基本上我所追求的是与下面的TSQL等效的LINQ

SELECT Users.ID, Users.UserName, Users.Name 
    FROM Users LEFT OUTER JOIN tblUserBusiness ON Users.ID = tblUserBusiness.UserID
    WHERE (Users.IsSysAdmin = 1) OR (tblUserBusiness.BUID = 5)

Answer 1

尝试一下：

Dim buID As Integer = ... ' BUID to get

Dim q = From u In Users
        Group Join bu In tblUserBusiness On u.Id Equals bu.UserID Into Group
        From j In Group.DefaultIfEmpty
        Where u.IsSysAdmin OrElse If(j IsNot Nothing, j.BUID = buID, False)
        Select u

...或...
Dim q = From u In Users
        Group Join bu In tblUserBusiness On u.Id Equals bu.UserID Into Group
        From j In Group.Where(Function(x) x.BUID = buID).DefaultIfEmpty
        Where u.IsSysAdmin OrElse j IsNot Nothing
        Select u

任何人都会给您您所需要的。我认为：）

Answer 2

您需要使用DefaultIfEmpty，所以应该是

Group Join bu In db.tblUserBusinesses.DefaultIfEmpty

这是一个简化的测试用例（需要一个全新的控制台应用程序：]
Module Module1
  Class Person
    Public Property Id As String
    Public Property FirstName As String
  End Class

  Class Address
    Public Property Id As String
    Public Property StreetName As String
  End Class

  Sub Main()
    Dim personList As New List(Of Person)
    With personList
      .Add(New Person With {.Id = "1", .FirstName = "John"})
      .Add(New Person With {.Id = "2", .FirstName = "Peter"})
      .Add(New Person With {.Id = "3", .FirstName = "Victor"})
    End With

    Dim addressList As New List(Of Address)
    With addressList
      .Add(New Address With {.Id = "1", .StreetName = "Baker Street"})
      .Add(New Address With {.Id = "2", .StreetName = "Broadway"})
      .Add(New Address With {.Id = "4", .StreetName = "Hwy 999"})
    End With

    Dim v = From p In personList
            Group Join a In addressList.DefaultIfEmpty
            On a.Id Equals p.Id Into Group
            Select PersonId = p.Id,
                   PersonName = p.FirstName,
                   StreetName = Group.FirstOrDefault
  End Sub
End Module

虽然这可能不是人与地址关系的理想设计，但它证明Outer Join可以使用DefaultIfEmpty和FirstOrDefault在VB.NET中工作（可选，可帮助您在漂亮时尚）。

Answer 3

+ 1为@ajakblackgoat答案。谢谢兄弟，它帮助了我！但是我是C＃开发人员，所以如果其他类似我的开发人员登陆此页面，则为他们发布

C＃版本 ：)

from u in Users
join bu in tblUserBusiness on u.Id equals bu.UserID into group
from j in group.DefaultIfEmpty()
where (u.IsSysAdmin || (j != null ? j.BUID == buID : false))

试图发表评论，但未正确显示完整代码