Starting in 9.3, Tableau introduced the ability to combine, or union, disparate data sources. This feature is handy as the data needed for analysis is frequently split across many different files. For example, suppose that you have two Microsoft Excel files that analyzed team wins data; the first file contains the 2016 season data while the second file contains the 2017 season data. Prior to 9.3, you'd have to combine these files outside of Tableau, which is less efficient and takes you out of the analytical flow. The good news is that this process is much easier with Tableau File Union!
This feature can be used for flat files such as MS Excel and text as well as with several common relational databases. In this case, simply connect to the first Excel file and add the desired sheet to the canvas; then convert that data source to a union.
Tableau displays a new dialog box, which allows the user to select between a specific and wildcard union. The specific union works well for combining sheets from the same workbook together while the wildcard union allows the user to combine disparate files together.
The wildcard union allows the user to select which workbooks as well as which worksheets contained in the defined workbooks should be combined. In the example above, the asterisk is used to instruct Tableau to combine any sheet that begins with a sheet name of "Team Win" from workbooks that have a name beginning with "201". By default, Tableau will look in the same file location as the original data source; however, this can be changed to look in subfolders and the parent folder.