Union and join basics in Tableau - Cell-by-Cell Formula Trace

Sample Data

Two tables for union (left and right) with customer info, and one table for join with sales data.

Cell	Value
A1	ID
B1	Name
C1	City
A2	1
B2	Alice
C2	New York
A3	2
B3	Bob
C3	Chicago
A4	3
B4	Charlie
C4	Seattle
E1	ID
F1	Name
G1	City
E2	4
F2	Diana
G2	Boston
E3	5
F3	Evan
G3	Denver
E4	6
F4	Fiona
G4	Austin
I1	ID
J1	Name
K1	City
L1	Sales
I2	1
J2	Alice
K2	New York
L2	100
I3	2
J3	Bob
K3	Chicago
L3	150
I4	7
J4	George
K4	Miami
L4	200

Formula Trace

UNION(Table1, Table2) and LEFT JOIN UNION(Table1, Table2) with SalesTable on ID

Step 1: UNION(Table1, Table2)

Step 2: LEFT JOIN (Result of UNION) with SalesTable on ID

Cell Reference Map

  A      B       C       E      F       G       I      J       K       L
+-----+-------+-------+-----+-------+-------+-----+-------+-------+-------+
| ID  | Name  | City  | ID  | Name  | City  | ID  | Name  | City  | Sales |
|  1  | Alice | New York |  4  | Diana | Boston|  1  | Alice | New York |  100  |
|  2  | Bob   | Chicago |  5  | Evan  | Denver|  2  | Bob   | Chicago |  150  |
|  3  | Charlie| Seattle | 6  | Fiona | Austin|  7  | George| Miami |  200  |
+-----+-------+-------+-----+-------+-------+-----+-------+-------+-------+
  ^       ^       ^       ^       ^       ^       ^       ^       ^       ^
  |       |       |       |       |       |       |       |       |       |
  +-------+-------+-------+-------+-------+-------+-------+-------+-------+
References: Table1 (A1:C4), Table2 (E1:G4), SalesTable (I1:L4)

Shows the three tables with their columns and rows. Arrows indicate these cells are referenced in union and join operations.

Result

  M      N       O       P
+-----+-------+-------+-------+
| ID  | Name  | City  | Sales |
|  1  | Alice | New York |  100  |
|  2  | Bob   | Chicago | 150  |
|  3  | Charlie | Seattle | null |
|  4  | Diana | Boston | null  |
|  5  | Evan  | Denver | null  |
|  6  | Fiona | Austin | null  |
+-----+-------+-------+-------+

Final table after union and left join. All customers from both tables appear with sales data where available; missing sales show null.

Sheet Trace Quiz - 3 Questions

What does the UNION operation do with the two tables?

A Stacks all rows from both tables vertically, keeping all columns

B Matches rows by ID and combines columns side by side

C Keeps only rows that appear in both tables

D Removes duplicate rows and columns

Key Result

UNION stacks rows vertically from two tables; LEFT JOIN adds columns from another table matching on key, keeping all left rows.

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