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.

Practice

1. Which of the following best describes a Union in Tableau?

easy

A. It filters data based on a condition.

B. It combines columns from two tables based on matching keys.

C. It creates a new calculated field.

D. It stacks rows from two or more tables with similar columns.

Solution

1. Step 1: Understand Union purpose

   A Union stacks rows from tables that have the same columns, combining data vertically.

2. Step 2: Compare with Join

   Join combines columns horizontally based on matching keys, not stacking rows.

3. Final Answer:

   It stacks rows from two or more tables with similar columns. -> Option D

4. Quick Check:

   Union = stacking rows [OK]

Hint: Union stacks rows; Join combines columns [OK]

Common Mistakes:

• Confusing Union with Join
• Thinking Union combines columns
• Assuming Union filters data

2. Which syntax correctly represents a Join in Tableau's data source page?

easy

A. Drag one table onto another and select the join type.

B. Use the UNION keyword in the data pane.

C. Create a calculated field with JOIN() function.

D. Right-click a table and select 'Append'.

Solution

1. Step 1: Identify Join method in Tableau

   In Tableau, you join tables by dragging one table onto another and choosing the join type (Inner, Left, Right, Full).

2. Step 2: Eliminate incorrect options

   Union keyword is for unions, JOIN() function does not exist, and 'Append' is not a Tableau option.

3. Final Answer:

   Drag one table onto another and select the join type. -> Option A

4. Quick Check:

   Join = drag tables + select join [OK]

Hint: Join by dragging tables together in Tableau [OK]

Common Mistakes:

• Confusing Union keyword with Join
• Looking for JOIN() function which doesn't exist
• Using 'Append' instead of Union or Join

3. Given two tables:
Table A: ID, Name
Table B: ID, Sales
What will be the result of an Inner Join on ID?

medium

A. A table with only IDs from Table A and no Sales column.

B. A table stacking all rows from both tables vertically.

C. A table with columns ID, Name, Sales containing only matching IDs.

D. A table with all IDs from both tables, including unmatched rows.

Solution

1. Step 1: Understand Inner Join behavior

   Inner Join returns rows where the join key (ID) exists in both tables, combining columns.

2. Step 2: Analyze columns and rows

   The result will have columns ID, Name, Sales and only rows with matching IDs in both tables.

3. Final Answer:

   A table with columns ID, Name, Sales containing only matching IDs. -> Option C

4. Quick Check:

   Inner Join = matching rows + combined columns [OK]

Hint: Inner Join keeps only matching keys with all columns [OK]

Common Mistakes:

• Thinking Inner Join stacks rows
• Expecting unmatched rows included
• Confusing columns included in result

4. You tried to Union two tables but got an error. What is the most likely cause?

medium

A. You forgot to select a join type.

B. The tables have different column names or data types.

C. You used a calculated field instead of a table.

D. You dragged tables instead of using the Union option.

Solution

1. Step 1: Identify Union requirements

   Union requires tables to have matching column names and compatible data types to stack rows.

2. Step 2: Understand error cause

   If columns differ in name or type, Tableau cannot stack rows, causing an error.

3. Final Answer:

   The tables have different column names or data types. -> Option B

4. Quick Check:

   Union error = mismatched columns/types [OK]

Hint: Union needs matching columns and types [OK]

Common Mistakes:

• Confusing Union errors with Join errors
• Not checking column names and types
• Trying to drag tables instead of using Union

5. You have monthly sales data split into two tables: Sales_Jan and Sales_Feb with identical columns. You also have a Customers table with customer details. How do you create a combined dataset showing all sales with customer info?

hard

A. Union Sales_Jan and Sales_Feb, then Join the result with Customers on CustomerID.

B. Join Sales_Jan with Customers, then Union with Sales_Feb.

C. Union Customers with Sales_Jan and Sales_Feb directly.

D. Join Sales_Jan and Sales_Feb on CustomerID, then Union with Customers.

Solution

1. Step 1: Combine monthly sales data vertically

   Since Sales_Jan and Sales_Feb have identical columns, use Union to stack their rows into one sales table.

2. Step 2: Enrich sales data with customer info

   Join the combined sales table with Customers on CustomerID to add customer details horizontally.

3. Final Answer:

   Union Sales_Jan and Sales_Feb, then Join the result with Customers on CustomerID. -> Option A

4. Quick Check:

   Union sales first, then Join customers [OK]

Hint: Union similar tables first, then Join related tables [OK]

Common Mistakes:

• Joining before unioning sales tables
• Trying to union unrelated tables
• Joining sales tables instead of unioning