
Getting Started with Tableau 2019.2
By :

If a join adds columns, a union adds rows.
Unions are useful when you have two (or more) tables with an identical structure (the same columns) that you want to combine to create a unique data source.
A typical use case is when you have a dataset that contains one table per year, and you need to compare those years. To do that, you need to combine those different tables into a single data source. You can, of course, spend some time copying and pasting the data into a new table, but with Tableau and unions, you can do this in a few clicks.
To create a union, the different tables must contain the same column names. Otherwise, Tableau will not consider them to be identical and will create new columns.
Let's start with an example.
For this example, I created an Excel file with two sheets to union.
If you want to reproduce the example, you can download the Union example
file from my website, https://tableau2019.ladataviz.com, in Chapter 4: Connect...