The CData Power BI Connector for Xero 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 a.Name, po.Total FROM Contacts a, PurchaseOrders po WHERE a.ContactId = po.Contact_ContactlId

Left Join

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

SELECT c.Name, po.Total FROM Contacts c LEFT JOIN PurchaseOrders po ON c.ContactId = po.Contact_ContactId