The CData Cmdlets PowerShell Module for ServiceNow supports standard SQL joins like the following examples.

Inner Join

An inner join selects only rows from both tables that match the join condition:

SELECT incident.close_notes, system_user.name FROM incident INNER JOIN system_user ON incident.caller_id = system_user.system_id

Left Join

A left join selects all rows in the FROM table and only matching rows in the JOIN table:

SELECT incident.close_notes, system_user.name FROM incident LEFT JOIN system_user ON incident.caller_id = system_user.system_id