Linq to objects | 易学教程

问题

I have 2 databases from different servers. I cannot link the databases. The data is retrieved from the databases as :

DB1

- Client_ID Engagement_ID Enabled Description

600 10 True Company1
600 20 False Company2
700 10 True Company3

DB2

Client_ID Engagement_ID Enabled Description
600 5 True Company1
600 10 False Company2
500 30 True Company3

The T SQL for this task is:

select * from DB1
left join DB2 on DB1.client_ID = DB2.client_ID
and DB1.Engagement_ID = DB2.Engagement_ID
where DB2.CLient_ID is null
and DB2.Engagement_ID is null and DB1.client_id in (select client_id from DB2)

I need to do this VB.NET LINQ

回答1:

The sample data you provided won't return any values anyway since they ClientId and EngagementId all have values.

I split the LINQ into two lists. I have yet to test this out or optimize it, but maybe this is what you're looking for to at least get you started.

Here is my attempt:

Public Class DBObject

    Public Sub New(ByVal cId As Integer, _
                   ByVal eId As Integer, _
                   ByVal enabled As Boolean, _
                   ByRef desc As String)

        _clientId = cId
        _engagementId = eId
        _enabled = enabled
        _description = desc

    End Sub

    Private _clientId As Integer
    Public Property ClientId() As Integer
        Get
            Return _clientId
        End Get
        Set(ByVal value As Integer)
            _clientId = value
        End Set
    End Property

    Private _engagementId As Integer
    Public Property EngagementId() As Integer
        Get
            Return _engagementId
        End Get
        Set(ByVal value As Integer)
            _engagementId = value
        End Set
    End Property

    Private _enabled As Boolean
    Public Property Enabled() As Boolean
        Get
            Return _enabled
        End Get
        Set(ByVal value As Boolean)
            _enabled = value
        End Set
    End Property

    Private _description As String
    Public Property Description() As String
        Get
            Return _description
        End Get
        Set(ByVal value As String)
            _description = value
        End Set
    End Property

End Class

Dim DB1 As New List(Of DBObject)
Dim DB2 As New List(Of DBObject)

DB1.Add(New DBObject(600, 10, True, "Company1"))
DB1.Add(New DBObject(600, 20, False, "Company2"))
DB1.Add(New DBObject(700, 10, True, "Company3"))

DB2.Add(New DBObject(600, 5, True, "Company1"))
DB2.Add(New DBObject(600, 10, False, "Company2"))
DB2.Add(New DBObject(500, 30, True, "Company3"))

Dim list1 As List(Of DBObject) = (From obj1 As DBObject In DB1 _
                                         Join obj2 As DBObject In DB2 _
                                         On obj1.ClientId Equals obj2.ClientId _
                                         And obj1.EngagementId Equals obj2.EngagementId _
                                         Where obj2.ClientId = Nothing _
                                         And obj2.EngagementId = Nothing _
                                         Select obj1).ToList

Dim list2 As List(Of DBObject) = (From obj3 As DBObject In list1 _
                                          From obj4 As DBObject In DB2 _
                                          Where obj3.ClientId = obj4.ClientId _
                                          Select obj3).ToList

' list2 would have the results you desire

回答2:

    Dim list1 = From obj1 As DBObject In DB1 _
                   Group Join obj2 As DBObject In DB2 _
                   On obj1.ClientId Equals obj2.ClientId _
                   And obj1.EngagementId Equals obj2.EngagementId _
                   Into g = Group _
                   From r In g.DefaultIfEmpty() _
                   Where g.ElementAtOrDefault(0) Is Nothing _
                   Select New With {.Cl_ID = obj1.ClientId, .EngID = bj1.EngagementId}  


    Dim list2 = (From obj3 In list1 _
                  From obj4 In DB2 _
                  Where obj3.Cl_ID = obj4.ClientId _
                  Select obj3).Distinct.DefaultIfEmpty

I modified sunpech's code and List2 contains the expected result i.e. the second row of DB1 - 600, 20, False, "Company2"

来源：https://stackoverflow.com/questions/3189631/linq-to-objects

标签

vb.net

linq-to-objects