Joining tables in Tableau - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a join in Tableau?

A join in Tableau combines rows from two or more tables based on a related column between them, allowing you to analyze combined data in one view.

beginner

Name the four main types of joins in Tableau.

Inner Join, Left Join, Right Join, and Full Outer Join.

beginner

What does an Inner Join do?

An Inner Join returns only the rows where there is a match in both tables based on the join key.

intermediate

Explain the difference between Left Join and Right Join.

A Left Join returns all rows from the left table and matching rows from the right table. A Right Join returns all rows from the right table and matching rows from the left table.

intermediate

What is a Full Outer Join in Tableau?

A Full Outer Join returns all rows from both tables. When there is no match, the result will have NULLs for missing values from either table.

Which join type returns only matching rows from both tables?

A Inner Join

B Left Join

C Right Join

D Full Outer Join

Inner Join returns only rows where the join keys match in both tables.

If you want all rows from the left table and matching rows from the right, which join do you use?

A Right Join

B Full Outer Join

C Left Join

D Inner Join

Left Join keeps all rows from the left table and adds matching rows from the right.

What happens to rows with no match in a Full Outer Join?

A They are excluded

B They appear duplicated

C They cause an error

D They appear with NULLs in missing columns

Full Outer Join includes all rows from both tables, filling missing values with NULLs.

In Tableau, where do you define the join between tables?

A In the Worksheet

B In the Data Source tab

C In the Dashboard

D In the Story

Joins are defined in the Data Source tab when connecting tables.

Which join type would you use to keep all data from both tables regardless of matches?

A Full Outer Join

B Left Join

C Right Join

D Inner Join

Full Outer Join keeps all rows from both tables, matching where possible.

Describe the four main types of joins in Tableau and when you might use each.

Think about which table's data you want to keep fully and how unmatched rows are handled.

Explain how you would join two tables in Tableau and what you should check to ensure the join works correctly.

Focus on the steps in Tableau interface and the importance of matching columns.

Practice

1. What is the main purpose of joining tables in Tableau?

easy

A. To export data to Excel

B. To create charts and graphs automatically

C. To filter data within a single table

D. To combine related data from different tables for analysis

Solution

1. Step 1: Understand the concept of joining tables

   Joining tables means combining data from two or more tables based on a related column.

2. Step 2: Identify the purpose in Tableau

   Tableau uses joins to bring together related data so you can analyze it as one set.

3. Final Answer:

   To combine related data from different tables for analysis -> Option D

4. Quick Check:

   Joining tables = combine related data [OK]

Hint: Joining means combining data from tables [OK]

Common Mistakes:

• Thinking joins create charts automatically
• Confusing joins with filtering data
• Assuming joins export data

2. Which of the following is the correct syntax to create an inner join between two tables Orders and Customers on the CustomerID field in Tableau's custom SQL?

easy

A. SELECT * FROM Orders INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID

B. SELECT * FROM Orders JOIN Customers WHERE Orders.CustomerID = Customers.CustomerID

C. SELECT * FROM Orders LEFT JOIN Customers USING CustomerID

D. SELECT * FROM Orders FULL JOIN Customers ON Orders.CustomerID = Customers.CustomerID

Solution

1. Step 1: Recall correct SQL join syntax

   The INNER JOIN syntax requires the ON keyword with the join condition.

2. Step 2: Check each option

   SELECT * FROM Orders INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID uses INNER JOIN with ON and correct condition; others use wrong keywords or join types.

3. Final Answer:

   SELECT * FROM Orders INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID -> Option A

4. Quick Check:

   INNER JOIN syntax = SELECT * FROM Orders INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID [OK]

Hint: INNER JOIN needs ON with condition, not WHERE [OK]

Common Mistakes:

• Using WHERE instead of ON for join condition
• Confusing join types (LEFT, FULL instead of INNER)
• Missing ON keyword

3. Given two tables:
Products with ProductID 1,2,3 and Sales with ProductID 2,3,4.
What will be the result count of rows after a left join Products LEFT JOIN Sales ON Products.ProductID = Sales.ProductID?

medium

A. 4 rows

B. 3 rows

C. 2 rows

D. 5 rows

Solution

1. Step 1: Understand left join behavior

   A left join keeps all rows from the left table (Products) and matches rows from the right (Sales).

2. Step 2: Count rows from Products

   Products has 3 rows (ProductID 1,2,3), so result will have 3 rows regardless of matches.

3. Final Answer:

   3 rows -> Option B

4. Quick Check:

   Left join rows = left table rows [OK]

Hint: Left join keeps all left table rows [OK]

Common Mistakes:

• Counting all unique keys from both tables
• Confusing left join with inner join
• Assuming unmatched rows add extra rows

4. You created a join between Orders and Customers on CustomerID, but your result shows fewer rows than expected. What is the most likely cause?

medium

A. You forgot to add a join condition

B. You used a left join which removes unmatched rows

C. You used an inner join but some CustomerIDs do not match

D. You joined on different field names with no relation

Solution

1. Step 1: Analyze join type impact

   Inner join returns only matching rows; unmatched rows are excluded.

2. Step 2: Identify cause of fewer rows

   If some CustomerIDs don't match, inner join reduces row count.

3. Final Answer:

   You used an inner join but some CustomerIDs do not match -> Option C

4. Quick Check:

   Inner join excludes unmatched rows [OK]

Hint: Inner join drops unmatched rows, reducing count [OK]

Common Mistakes:

• Thinking left join removes unmatched rows
• Ignoring join condition importance
• Assuming join always increases rows

5. You have three tables: Orders, Customers, and Regions. You want to create a report showing total sales by region. Which join sequence in Tableau is best to ensure all orders are included even if some customers or regions are missing?

hard

A. Orders LEFT JOIN Customers ON CustomerID, then LEFT JOIN Regions ON RegionID

B. Customers INNER JOIN Orders ON CustomerID, then INNER JOIN Regions ON RegionID

C. Regions RIGHT JOIN Customers ON RegionID, then INNER JOIN Orders ON CustomerID

D. Orders INNER JOIN Customers ON CustomerID, then LEFT JOIN Regions ON RegionID

Solution

1. Step 1: Understand requirement to include all orders

   We want all orders even if customer or region info is missing, so start with Orders as left table.

2. Step 2: Choose join types to keep all orders

   Using LEFT JOIN from Orders to Customers keeps all orders; then LEFT JOIN to Regions keeps all orders even if region missing.

3. Final Answer:

   Orders LEFT JOIN Customers ON CustomerID, then LEFT JOIN Regions ON RegionID -> Option A

4. Quick Check:

   Left joins keep all left table rows [OK]

Hint: Use left joins starting from main table to keep all data [OK]

Common Mistakes:

• Using inner joins that drop unmatched orders
• Joining in wrong sequence losing data
• Using right join confusing left table priority