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Join Clause (Visual Basic)

Combines two collections into a single collection. The join operation is based on matching keys and uses the Equals operator.

Join element In collection _
  [ joinClause _ ] 
  [ groupJoinClause ... _ ] 
On key1 Equals key2 [ And key3 Equals key4 [... ]
element

Required. The control variable for the collection being joined.

collection

Required. The collection to combine with the collection identified on the left side of the Join operator. A Join clause can be nested in another Join clause, or in a Group Join clause.

joinClause

Optional. One or more additional Join clauses to further refine the query.

groupJoinClause

Optional. One or more additional Group Join clauses to further refine the query.

key1Equalskey2

Required. Identifies keys for the collections being joined. You must use the Equals operator to compare keys from the collections being joined. You can combine join conditions by using the And operator to identify multiple keys. key1 must be from the collection on the left side of the Join operator. key2 must be from the collection on the right side of the Join operator.

The keys used in the join condition can be expressions that include more than one item from the collection. However, each key expression can contain only items from its respective collection.

The Join clause combines two collections based on matching key values from the collections being joined. The resulting collection can contain any combination of values from the collection identified on the left side of the Join operator and the collection identified in the Join clause. The query will return only results for which the condition specified by the Equals operator is met. This is equivalent to an INNER JOIN in SQL.

You can use multiple Join clauses in a query to join two or more collections into a single collection.

You can perform an implicit join to combine collections without the Join clause. To do this, include multiple In clauses in your From clause and specify a Where clause that identifies the keys that you want to use for the join.

You can use the Group Join clause to combine collections into a single hierarchical collection. This is like a LEFT OUTER JOIN in SQL.

The following code example performs an implicit join to combine a list of customers with their orders.


Dim customerIDs() = {"ALFKI", "VICTE", "BLAUS", "TRAIH"}

Dim customerList = From cust In customers, custID In customerIDs
                   Where cust.CustomerID = custID
                   Select cust.CompanyName

For Each companyName In customerList
  Console.WriteLine(companyName)
Next


The following code example joins two collections by using the Join clause.


Imports System.Diagnostics
Imports System.Security.Permissions

Public Class JoinSample

  <SecurityPermission(SecurityAction.Demand)>
  Public Sub ListProcesses()
    Dim processDescriptions As New List(Of ProcessDescription)
    processDescriptions.Add(New ProcessDescription With {
                                    .ProcessName = "explorer",
                                    .Description = "File Explorer"})
    processDescriptions.Add(New ProcessDescription With {
                                .ProcessName = "winlogon",
                                .Description = "Windows Logon"})
    processDescriptions.Add(New ProcessDescription With {
                                .ProcessName = "cmd",
                                .Description = "Command Window"})
    processDescriptions.Add(New ProcessDescription With {
                                .ProcessName = "iexplore",
                                .Description = "Internet Explorer"})

    Dim processes = From proc In Process.GetProcesses
                    Join desc In processDescriptions
                      On proc.ProcessName Equals desc.ProcessName
                    Select proc.ProcessName, proc.Id, desc.Description

    For Each proc In processes
      Console.WriteLine("{0} ({1}), {2}",
                        proc.ProcessName, proc.Id, proc.Description)
    Next
  End Sub

End Class

Public Class ProcessDescription
  Public ProcessName As String
  Public Description As String
End Class


This example will produce output similar to the following:

winlogon (968), Windows Logon

explorer (2424), Windows Explorer

cmd (5136), Command Window

The following code example joins two collections by using the Join clause with two key columns.


Imports System.Diagnostics
Imports System.Security.Permissions

Public Class JoinSample2

  <SecurityPermission(SecurityAction.Demand)>
  Public Sub ListProcesses()
    Dim processDescriptions As New List(Of ProcessDescription2)

    ' 8 = Normal priority, 13 = High priority
    processDescriptions.Add(New ProcessDescription2 With {
                                    .ProcessName = "explorer",
                                    .Description = "File Explorer",
                                    .Priority = 8})
    processDescriptions.Add(New ProcessDescription2 With {
                                .ProcessName = "winlogon",
                                .Description = "Windows Logon",
                                .Priority = 13})
    processDescriptions.Add(New ProcessDescription2 With {
                                .ProcessName = "cmd",
                                .Description = "Command Window",
                                .Priority = 8})
    processDescriptions.Add(New ProcessDescription2 With {
                                .ProcessName = "iexplore",
                                .Description = "Internet Explorer",
                                .Priority = 8})

    Dim processes = From proc In Process.GetProcesses
                    Join desc In processDescriptions
                      On proc.ProcessName Equals desc.ProcessName And 
                         proc.BasePriority Equals desc.Priority
                    Select proc.ProcessName, proc.Id, desc.Description,
                           desc.Priority

    For Each proc In processes
      Console.WriteLine("{0} ({1}), {2}, Priority = {3}",
                        proc.ProcessName,
                        proc.Id,
                        proc.Description,
                        proc.Priority)
    Next
  End Sub

End Class

Public Class ProcessDescription2
  Public ProcessName As String
  Public Description As String
  Public Priority As Integer
End Class


The example will produce output similar to the following:

winlogon (968), Windows Logon, Priority = 13

cmd (700), Command Window, Priority = 8

explorer (2424), Windows Explorer, Priority = 8

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