Union and join basics in Tableau - Step-by-Step Guide

Introduction

This feature helps you combine data from multiple tables or files in Tableau. You can stack data with a union or connect data side-by-side with a join. It solves the problem of working with related data spread across different sources.

When you have monthly sales data in separate Excel files and want to analyze all months together.

When you want to combine customer information from two different databases into one view.

When you need to add product details to your sales transactions by matching product IDs.

When you want to see all records from two tables, even if some don’t match.

When you want to filter or analyze data that comes from different sources but shares common fields.

Steps

Step 1: Open Tableau and connect to your first data source

- Start page → Connect pane

The data source appears in the Data pane on the left

💡 Use Excel, CSV, or database connections depending on your data

Step 2: Drag the first table or file to the canvas in the Data Source tab

- Data Source tab → canvas area

The table appears as a data grid on the canvas

Step 3: To create a union, drag the second table or file directly below the first one until you see a 'Union' option

- Data Source tab → canvas area below first table

Tables stack vertically and show combined rows from both tables

💡 Tables must have similar columns for a clean union

Step 4: To create a join, drag the second table next to the first one on the canvas

- Data Source tab → canvas area next to first table

A join icon appears between tables showing how they connect

💡 Select the join type (Inner, Left, Right, Full Outer) by clicking the join icon

Step 5: Click the join icon to set join keys by selecting matching fields from each table

- Join configuration popup

Tables connect based on the selected fields, filtering data accordingly

Step 6: Review the preview data grid to confirm the union or join worked as expected

- Data Source tab → data preview area

You see combined data rows for union or merged columns for join

Before vs After

Before

Two separate tables: one with 100 sales records, another with 50 product details

After

One combined table showing 150 rows stacked (union) or 100 rows with product info added (join)

Settings Reference

Join Type

📍 Join icon on Data Source canvas

Controls which rows appear based on matching keys between tables

Default: Inner

Union Tables

📍 Drag tables vertically on Data Source canvas

Stacks tables to combine rows from multiple sources

Default: Manual union by dragging

Join Keys

📍 Join configuration popup

Defines how tables relate by matching column values

Default: None

Common Mistakes

Trying to union tables with different column names or orders

Union stacks rows but requires similar columns to align data correctly

Rename or reorder columns in source files to match before union

Using the wrong join type and losing data unintentionally

Inner join removes unmatched rows, which may hide important data

Choose Left or Full Outer join if you want to keep all records from one or both tables

Not selecting join keys correctly, causing incorrect data matches

Join keys must match the fields that relate the tables logically

Carefully select matching columns like Customer ID or Product ID for join keys

Summary

Union stacks data tables vertically to combine rows with similar columns.

Join connects tables side-by-side based on matching fields to combine columns.

Choose the right join type and matching keys to avoid losing or mismatching data.