How to: Combine Data with LINQ by Using Joins (Visual Basic)

Article
11/16/2012

Visual Basic provides the Join and Group Join query clauses to enable you to combine the contents of multiple collections based on common values between the collections. These values are known as key values. Developers familiar with relational database concepts will recognize the Join clause as an INNER JOIN and the Group Join clause as, effectively, a LEFT OUTER JOIN.

The examples in this topic demonstrate a few ways to combine data by using the Join and Group Join query clauses.

Create a Project and Add Sample Data

To create a project that contains sample data and types

To run the samples in this topic, open Visual Studio and add a new Visual Basic Console Application project. Double-click the Module1.vb file created by Visual Basic.

The samples in this topic use the Person and Pet types and data from the following code example. Copy this code into the default Module1 module created by Visual Basic.

Private _people As List(Of Person)
Private _pets As List(Of Pet)

Function GetPeople() As List(Of Person)
  If _people Is Nothing Then CreateLists()
  Return _people
End Function

Function GetPets(ByVal people As List(Of Person)) As List(Of Pet)
  If _pets Is Nothing Then CreateLists()
  Return _pets
End Function

Private Sub CreateLists()
  Dim pers As Person

  _people = New List(Of Person)
  _pets = New List(Of Pet)

  pers = New Person With {.FirstName = "Magnus", .LastName = "Hedlund"}
  _people.Add(pers)
  _pets.Add(New Pet With {.Name = "Daisy", .Owner = pers})

  pers = New Person With {.FirstName = "Terry", .LastName = "Adams"}
  _people.Add(pers)
  _pets.Add(New Pet With {.Name = "Barley", .Owner = pers})
  _pets.Add(New Pet With {.Name = "Boots", .Owner = pers})
  _pets.Add(New Pet With {.Name = "Blue Moon", .Owner = pers})

  pers = New Person With {.FirstName = "Charlotte", .LastName = "Weiss"}
  _people.Add(pers)
  _pets.Add(New Pet With {.Name = "Whiskers", .Owner = pers})

  ' Add a person with no pets for the sake of Join examples.
  _people.Add(New Person With {.FirstName = "Arlene", .LastName = "Huff"})

  pers = New Person With {.FirstName = "Don", .LastName = "Hall"}
  ' Do not add person to people list for the sake of Join examples.
  _pets.Add(New Pet With {.Name = "Spot", .Owner = pers})

  ' Add a pet with no owner for the sake of Join examples.
  _pets.Add(New Pet With {.Name = "Unknown", _
                          .Owner = New Person With {.FirstName = String.Empty, _
                                                    .LastName = String.Empty}})
End Sub


...


Class Person
  Public _firstName As String
  Public _lastName As String

  Public Property FirstName() As String
    Get
      Return _firstName
    End Get
    Set(ByVal value As String)
      _firstName = value
    End Set
  End Property

  Public Property LastName() As String
    Get
      Return _lastName
    End Get
    Set(ByVal value As String)
      _lastName = value
    End Set
  End Property
End Class

Class Pet
  Public _name As String
  Public _owner As Person

  Public Property Name() As String
    Get
      Return _name
    End Get
    Set(ByVal value As String)
      _name = value
    End Set
  End Property

  Public Property Owner() As Person
    Get
      Return _owner
    End Get
    Set(ByVal value As Person)
      _owner = value
    End Set
  End Property
End Class

Perform an Inner Join by Using the Join Clause

An INNER JOIN combines data from two collections. Items for which the specified key values match are included. Any items from either collection that do not have a matching item in the other collection are excluded.

In Visual Basic, LINQ provides two options for performing an INNER JOIN: an implicit join and an explicit join.

An implicit join specifies the collections to be joined in a From clause and identifies the matching key fields in a Where clause. Visual Basic implicitly joins the two collections based on the specified key fields.

You can specify an explicit join by using the Join clause when you want to be specific about which key fields to use in the join. In this case, a Where clause can still be used to filter the query results.

To perform an Inner Join by using the Join clause

Add the following code to the Module1 module in your project to see examples of both an implicit and explicit inner join.

Sub InnerJoinExample()
  ' Create two lists. 
  Dim people = GetPeople()
  Dim pets = GetPets(people)

  ' Implicit Join. 
  Dim petOwners = From pers In people, pet In pets _
                  Where pet.Owner Is pers _
                  Select pers.FirstName, PetName = pet.Name

    ' Display grouped results. 
  Dim output As New System.Text.StringBuilder
  For Each pers In petOwners
    output.AppendFormat( _
      pers.FirstName & ":" & vbTab & pers.PetName & vbCrLf)
  Next

  Console.WriteLine(output)

  ' Explicit Join. 
  Dim petOwnersJoin = From pers In people _
                      Join pet In pets _
                      On pet.Owner Equals pers _
                      Select pers.FirstName, PetName = pet.Name

  ' Display grouped results.
  output = New System.Text.StringBuilder()
  For Each pers In petOwnersJoin
    output.AppendFormat( _
      pers.FirstName & ":" & vbTab & pers.PetName & vbCrLf)
  Next

  Console.WriteLine(output)

  ' Both queries produce the following output: 
  ' 
  ' Magnus:    Daisy 
  ' Terry:     Barley 
  ' Terry:     Boots 
  ' Terry:     Blue Moon 
  ' Charlotte: Whiskers 
End Sub

Perform a Left Outer Join by Using the Group Join Clause

A LEFT OUTER JOIN includes all the items from the left-side collection of the join and only matching values from the right-side collection of the join. Any items from the right-side collection of the join that do not have a matching item in the left-side collection are excluded from the query result.

The Group Join clause performs, in effect, a LEFT OUTER JOIN. The difference between what is typically known as a LEFT OUTER JOIN and what the Group Join clause returns is that the Group Join clause groups results from the right-side collection of the join for each item in the left-side collection. In a relational database, a LEFT OUTER JOIN returns an ungrouped result in which each item in the query result contains matching items from both collections in the join. In this case, the items from the left-side collection of the join are repeated for each matching item from the right-side collection. You will see what this looks like when you complete the next procedure.

You can retrieve the results of a Group Join query as an ungrouped result by extending your query to return an item for each grouped query result. To accomplish this, you have to ensure that you query on the DefaultIfEmpty method of the grouped collection. This ensures that items from the left-side collection of the join are still included in the query result even if they have no matching results from the right-side collection. You can add code to your query to provide a default result value when there is no matching value from the right-side collection of the join.

To perform a Left Outer Join by using the Group Join clause

Add the following code to the Module1 module in your project to see examples of both a grouped left outer join and an ungrouped left outer join.

Sub LeftOuterJoinExample()
  ' Create two lists. 
  Dim people = GetPeople()
  Dim pets = GetPets(people)

  ' Grouped results. 
  Dim petOwnersGrouped = From pers In people _
                         Group Join pet In pets _
                           On pers Equals pet.Owner _
                         Into PetList = Group _
                         Select pers.FirstName, pers.LastName, _
                                PetList

  ' Display grouped results. 
  Dim output As New System.Text.StringBuilder
  For Each pers In petOwnersGrouped
    output.AppendFormat(pers.FirstName & ":" & vbCrLf)
    For Each pt In pers.PetList
      output.AppendFormat(vbTab & pt.Name & vbCrLf)
    Next 
  Next

  Console.WriteLine(output)

  ' "Flat" results. 
  Dim petOwners = From pers In people _
                  Group Join pet In pets On pers Equals pet.Owner _
                  Into PetList = Group _
                  From pet In PetList.DefaultIfEmpty() _
                  Select pers.FirstName, pers.LastName, _
                         PetName = _
                           If(pet Is Nothing, String.Empty, pet.Name)
  ' This code produces the following output: 
  ' 
  ' Magnus: 
  '     Daisy 
  ' Terry: 
  '     Barley 
  '     Boots 
  '     Blue Moon 
  ' Charlotte: 
  '     Whiskers 
  ' Arlene: 


  ' Display "flat" results.
  output = New System.Text.StringBuilder()
  For Each pers In petOwners
    output.AppendFormat( _
      pers.FirstName & ":" & vbTab & pers.PetName & vbCrLf)
  Next

  Console.WriteLine(output.ToString())
End Sub 

' This code produces the following output: 
' 
' Magnus:       Daisy 
' Terry:        Barley 
' Terry:        Boots 
' Terry:        Blue Moon 
' Charlotte:    Whiskers 
' Arlene:

Perform a Join by Using a Composite Key

You can use the And keyword in a Join or Group Join clause to identify multiple key fields to use when matching values from the collections being joined. The And keyword specifies that all specified key fields must match for items to be joined.

To perform a Join by using a composite key

Add the following code to the Module1 module in your project to see examples of a join that uses a composite key.

Sub CompositeKeyJoinExample()
  ' Create two lists. 
  Dim people = GetPeople()
  Dim pets = GetPets(people)

  ' Implicit Join. 
  Dim petOwners = From pers In people _
                  Join pet In pets On _
                    pet.Owner.FirstName Equals pers.FirstName _
                    And pet.Owner.LastName Equals pers.LastName _
                  Select pers.FirstName, PetName = pet.Name

  ' Display grouped results. 
  Dim output As New System.Text.StringBuilder
  For Each pers In petOwners
    output.AppendFormat( _
      pers.FirstName & ":" & vbTab & pers.PetName & vbCrLf)
  Next

  Console.WriteLine(output)
End Sub

Run the Code

To add code to run the examples

Replace the Sub Main in the Module1 module in your project with the following code to run the examples in this topic.

Sub Main()
  InnerJoinExample()
  LeftOuterJoinExample()
  CompositeKeyJoinExample()

  Console.ReadLine()
End Sub

Press F5 to run the examples.

Change History

Date	History	Reason
September 2008	Added link.	Customer feedback.