The CData ADO.NET Provider for CSV 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 prod.Id, prod.Name, det.Details FROM Products prod INNER JOIN ProductDetails det ON prod.Id = det.ProductId

Left Join

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

 SELECT prod.Id, prod.Name, det.Details FROM Products prod LEFT JOIN ProductDetails det ON prod.Id = det.ProductId