Tableau Prep Unions
Unions in Tableau Prep work the same way as in Tableau Desktop – by placing one dataset on top of the other based on field name, and where field names do not match, records are filled using null. Columns also need to be the same data type to be unioned on top of each other.
To create a union, drag one step in the flow onto another so that the Join and Union popups appear. Then, drop the step on Union. This will create the Union logical step, which has a setting pane on the left-hand side that includes an inputs legend at the top. The legend lists all of the data sources included in the union (there can be up to 10, with new ones being added by dragging and dropping onto the union step), and each distinct step is color-coded. There is then a description of the total number of fields resulting from the union and the amount of them that are mismatched, meaning not included in all data sources in the union. Mismatched fields are then listed below with a square...