The CData Cmdlets PowerShell Module for Exact Online 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 Accounts.Name, Contacts.FullName FROM Accounts, Contacts WHERE Accounts.ID=Contacts.Account

Left Join

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

SELECT Accounts.Name, Contacts.FullName FROM Accounts LEFT OUTER JOIN Contacts ON Accounts.ID=Contacts.Account