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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.
Syntax
Join element In collection _
[ joinClause _ ]
[ groupJoinClause ... _ ]
On key1 Equals key2 [ And key3 Equals key4 [... ] ]
Parts
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.
key1 Equals key2
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.
Remarks
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.
Example 1
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
Example 2
The following code example joins two collections by using the Join clause.
Imports System.Diagnostics
Public Class JoinSample
Public Sub ListProcesses()
Dim processDescriptions As New List(Of ProcessDescription)
processDescriptions.Add(New ProcessDescription With {
.ProcessName = "explorer",
.Description = "Windows 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), File Explorer
cmd (5136), Command Window
Example 3
The following code example joins two collections by using the Join clause with two key columns.
Imports System.Diagnostics
Public Class JoinSample2
Public Sub ListProcesses()
Dim processDescriptions As New List(Of ProcessDescription2)
' 8 = Normal priority, 13 = High priority
processDescriptions.Add(New ProcessDescription2 With {
.ProcessName = "explorer",
.Description = "Windows 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), File Explorer, Priority = 8
See also
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