With Tableau, you have the ability to join (at a row level) across multiple different data connections. Joining across different data connections is referred to as a cross database join. For example, you can join SQL Server tables with text files or Excel files, or tables in one database with tables in another, even if those are on a different server. This opens up all kinds of possibilities for supplementing your data or analyzing data from disparate sources.

Consider the hospital data mentioned previously. It would not be uncommon for billing data to be in a separate system from patient care data. Let's say you had a file for patient billing that contained data you wanted to include in your analysis of hospital visits. You would be able to accomplish this by adding the text file as a data connection and then joining it to the existing tables, as follows:

You'll notice that the interface on the Data Source screen includes an Add link that allows you to add data connections to a data source. Clicking on each connection will allow you to drag and drop tables from that connection into the Data Source designer and specify the joins as you desire. Each data connection will be color-coded so that you can immediately identify the source of various tables in the designer.

In the preceding example, the Patient Billing.txt text file has been joined to the Patient table from SQL Server.