The CData Cmdlets PowerShell Module for ActiveCampaign 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 Deals.Id, Deals.ActivityCount, Deals.Currency, Deals.CustomerAccount FROM Deals INNER JOIN Accounts ON Deals.AccountId = Accounts.Id

Left Join

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

 SELECT Deals.Id, Deals.ActivityCount, Deals.Currency, Deals.CustomerAccount FROM Deals LEFT JOIN Accounts ON Deals.AccountId = Accounts.Id