The CData JDBC Driver 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