Difference between joining and blending data
Joining your data can only be done when the data comes from the same source, for example from two sheet tabs within a single Excel file. If that same information was stored in separate Excel files you would need to do a data blend in Tableau. A blend is always required if the data is stored in two separate “data sources” within Tableau. So even if your data is very closely related and exists in two separate files or databases, you will have to do a data blend if you are combining the data in Tableau.
When blending data, the first data source used in your view will dictate how your worksheet view in Tableau is built. The secondary (blended) data source will be able to contribute extra information, but will not be able to change the overall structure of the view. The secondary data source’s values can be aggregated and applied to the existing view after you have established a “relationship” by assigning a variable that both the primary and secondary data sources have in common.