In Powershell, what's the best way to join two tables into one?

I'm fairly new to Powershell, and am wondering if someone knows of any better way to accomplish the following example problem.

I have an array of mappings from IP address to host-name. This represents a list of active DHCP leases:

PS H:\> $leases

IP                    Name
--                    ----
192.168.1.1           Apple
192.168.1.2           Pear
192.168.1.3           Banana
192.168.1.99          FishyPC

I have another array of mappings from MAC address to IP address. This represents a list of IP reservations:

PS H:\> $reservations

IP                    MAC
--                    ---
192.168.1.1           001D606839C2
192.168.1.2           00E018782BE1
192.168.1.3           0022192AF09C
192.168.1.4           0013D4352A0D

For convenience, I was able to produce a third array of mappings from MAC address to IP address and host name using the following code. The idea is that $reservations should get a third field, "Name", which is populated whenever there's a matching "IP" field:

$reservations = $reservations | foreach {
    $res = $_
    $match = $leases | where {$_.IP -eq $res.IP} | select -unique
    if ($match -ne $NULL) {
        "" | select @{n="IP";e={$res.IP}}, @{n="MAC";e={$res.MAC}}, @{n="Name";e={$match.Name}}
    }
}

The desired output is something like this:

PS H:\> $ideal

IP                    MAC                 Name
--                    ---                 ----
192.168.1.1           001D606839C2        Apple
192.168.1.2           00E018782BE1        Pear
192.168.1.3           0022192AF09C        Banana
192.168.1.4           0013D4352A0D

Is there any better way of doing this?

After 1.5 years, the cmdlet I had pasted in the original answer has undergone so many updates that it has become completely outdated. Therefore I have replaced the code and the ReadMe with a link to the latest version.

Join-Object

Combines two object lists based on a related property between them.

Description
Combines properties from one or more objects. It creates a set that can be saved as a new object or used as it is. An object join is a means for combining properties from one (self-join) or more object lists by using values common to each.

Main features

Intuitive (SQL like) syntax
Smart property merging
Predefined join commands for updating, merging and specific join types
Well defined pipeline for the (left) input objects and output objects (preserves memory when correctly used)
Performs about 40% faster than Compare-Object on large object lists
Supports (custom) objects, data tables and dictionaries (e.g. hash tables) for input
Smart properties and calculated property expressions
Custom relation expressions
Easy installation (dot-sourcing)
Supports PowerShell for Windows (5.1) and PowerShell Core

The Join-Object cmdlet reveals the following proxy commands with their own (-JoinType and -Property) defaults:

InnerJoin-Object (Alias InnerJoin or Join), combines the related objects
LeftJoin-Object (Alias LeftJoin), combines the related objects and adds the rest of the left objects
RightJoin-Object (Alias RightJoin), combines the related objects and adds the rest of the right objects
FullJoin-Object (Alias FullJoin), combines the related objects and adds the rest of the left and right objects
CrossJoin-Object (Alias CrossJoin), combines each left object with each right object
Update-Object (Alias Update), updates the left object with the related right object
Merge-Object (Alias Merge), updates the left object with the related right object and adds the rest of the new (unrelated) right objects

ReadMe

The full ReadMe (and source code) is available from GitHub: https://github.com/iRon7/Join-Object

Installation

There are two versions of this Join-Object cmdlet (both versions supply the same functionality):

Join Module

Install-Module -Name JoinModule

Join Script

Install-Script -Name Join

(or rename the Join.psm1 module to a Join.ps1 script file)
and invoked the script by dot sourcing:

. .\Join.ps1

Answer

To answer the actual example in the question:

$reservations |LeftJoin $leases -On IP

IP          MAC          Name
--          ---          ----
192.168.1.1 001D606839C2 Apple
192.168.1.2 00E018782BE1 Pear
192.168.1.3 0022192AF09C Banana
192.168.1.4 0013D4352A0D

Performance

A little word on performance measuring:
The PowerShell pipeline is designed to stream objects (which safes memory), meaning that both¹ lists of input objects usually aren't (shouldn't be) resident in memory. Normally they are retrieved from somewhere else (i.e. a remote server, a disk). Also, the output usually matters where linq solutions are fast but might easily put you on the wrong foot in drawing conclusions because linq literally defers the execution (lazy evaluation), see also: fastest way to get a uniquely index item from the property of an array.
In other words, if it comes to (measuring) performance in PowerShell, it is important to look to the complete end-to-end solution, which is more likely to look something like:

 import-csv .\reservations.csv |LeftJoin (import-csv .\leases.csv) -On IP |Export-Csv .\results.csv

_{(1) Note: unfortunately, there is no easy way to build two parallel input streams (see: #15206 Deferred input pipelines)}

(more) Examples

More examples can be found in the related Stackoverflow questions at:

Combining Multiple CSV Files
Combine two CSVs - Add CSV as another Column
CMD or Powershell command to combine (merge) corresponding lines from two files
Can I use SQL commands (such as join) on objects in powershell, without any SQL server/database involved?
CMD or Powershell command to combine (merge) corresponding lines from two files
Compare Two CSVs, match the columns on 2 or more Columns, export specific columns from both csvs with powershell
Merge two CSV files while adding new and overwriting existing entries
Merging two CSVs and then re-ordering columns on output
Merge two CSV files while adding new and overwriting existing entries
Efficiently merge large object datasets having multiple matching keys
Is there a PowerShell equivalent of paste (i.e., horizontal file concatenation)?
combine (merge) corresponding lines from two files

And in the Join-Object test script.

_{Please give a 👍 if you support the proposal to Add a Join-Object cmdlet to the standard PowerShell equipment (#14994)}

This can also be done using my module Join-Object

Install-Module 'Join-Object'

Join-Object -Left $leases -Right $reservations -LeftJoinProperty 'IP' -RightJoinProperty 'IP'

Regarding performance, I tested against a sample data of 100k lines:

Hashtable example posted by @js2010 run in 8 seconds.
Join-Object by me run in 14 seconds.
LeftJoin by @iRon run in 1 minute and 50 seconds

Here's a simple example using a hashtable. With big arrays, this turns out to be faster.

$leases =
'IP,Name
192.168.1.1,Apple
192.168.1.2,Pear
192.168.1.3,Banana
192.168.1.99,FishyPC' | convertfrom-csv

$reservations =
'IP,MAC
192.168.1.1,001D606839C2
192.168.1.2,00E018782BE1
192.168.1.3,0022192AF09C
192.168.1.4,0013D4352A0D' | convertfrom-csv

$hashRes=@{}
foreach ($resRecord in $reservations) {
  $hashRes[$resRecord.IP] = $resRecord
}

$leases | foreach {
  $other = $hashRes[$_.IP]

  [pscustomobject]@{IP=$_.IP
                   MAC=$other.MAC
                  Name=$_.name}
}

IP           MAC          Name
--           ---          ----
192.168.1.1  001D606839C2 Apple
192.168.1.2  00E018782BE1 Pear
192.168.1.3  0022192AF09C Banana
192.168.1.99              FishyPC