The CData ADO.NET Provider for Avalara 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 c.Date, o.ItemCode, o.IsItemTaxable, o.Description FROM Transactions c INNER JOIN TransactionLines o ON c.Id = o.TransactionId

Left Join

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

SELECT c.ContactName, o.CustomerCode, o.Name, o.Value FROM Customers c LEFT JOIN CustomerCustomFields o ON c.CustomerCode = o.CustomerCode Where c.CompanyId = '247015' And o.CompanyId = '247015'