The CData ADO.NET Provider for QuickBooks 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 i.DocNumber, c.CompanyName FROM Invoices i INNER JOIN Customers c ON i.CustomerRef=c.Id

Left Join

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

SELECT i.DocNumber, c.CompanyName FROM Invoices i LEFT JOIN Customers c ON i.CustomerRef=c.Id