Most organizations have data sources that contain interrelated data; for example, before customers purchase products, you may have to prepare estimates. After they place their orders, you have to prepare invoices. And, of course, you need to know where to send the products. The estimate, the invoice, and the customer contact all contain related data. In the Data Source Library, you can combine two or more related data sources so that you can expose the data in one Data View.
SharePoint Designer provides you with two options for combining related data:
Merge. Use to combine data sources with the same set of fields; for example, you may store invoice data in many locations.
Join. Use to combine data sources that have one field ...