Union and join basics in Tableau - Dashboard Guide

Dashboard Mode - Union and join basics

Business Question

How can we combine sales data from two different regions to see total sales and compare product performance across both?

Sample Data

Sales_North

OrderID	Product	Sales
101	Widget	100
102	Gadget	150
103	Doohickey	200

Sales_South

OrderID	Product	Sales
201	Widget	120
202	Gadget	130
203	Thingamajig	180

Dashboard Components

• KPI Card: Total Sales Combined
  Formula: SUM([Sales]) after union of both tables
  Result: 880 (100+150+200+120+130+180)
• Bar Chart: Sales by Product
  Data: Union of Sales_North and Sales_South
  Aggregation: SUM(Sales) grouped by Product
  Shows total sales per product across both regions:
  - Widget: 220
  - Gadget: 280
  - Doohickey: 200
  - Thingamajig: 180
• Table: Joined Data on Product
  Join Type: Full Outer Join Sales_North to Sales_South on Product
  Columns:
  - Product
  - Sales_North (Sales from North region)
  - Sales_South (Sales from South region)
  Example rows:
  Widget | 100 | 120
  Gadget | 150 | 130
  Doohickey | 200 | NULL
  Thingamajig | NULL | 180

Dashboard Layout

+----------------------+--------------------------+
|  KPI: Total Sales    |  Bar Chart: Sales by     |
|  Combined (880)      |  Product                 |
+----------------------+--------------------------+
|          Table: Joined Data on Product           |
|  (Product, Sales_North, Sales_South)             |
+--------------------------------------------------+

Interactivity

A filter on Product lets users select one or more products. When filtered:

• The KPI card updates total sales for selected products only.
• The bar chart shows sales bars only for selected products.
• The joined table shows sales data only for selected products, with NULLs where data is missing.

This helps compare sales across regions for chosen products.

Self Check

If you add a filter to show only Product = 'Widget', which components update and what results do they show?

• KPI Card: Shows total sales 220 (100 North + 120 South)
• Bar Chart: Shows one bar for Widget with height 220
• Joined Table: Shows one row:
  Widget | 100 | 120

Key Result

Dashboard combining sales data from two regions using union and join to compare total and product-level sales.

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