Joining tables in Tableau - Cell-by-Cell Formula Trace

Sample Data

Two tables: Orders (A1:C4) and Customers (E1:G4). Orders has order details with CustomerID. Customers has customer info with CustomerID.

Cell	Value
A1	OrderID
B1	CustomerID
C1	OrderDate
A2	1001
B2	C01
C2	2024-01-10
A3	1002
B3	C02
C3	2024-01-12
A4	1003
B4	C03
C4	2024-01-15
E1	CustomerID
F1	CustomerName
G1	Country
E2	C01
F2	Alice
G2	USA
E3	C02
F3	Bob
G3	Canada
E4	C04
F4	Diana
G4	UK

Formula Trace

JOIN Orders and Customers ON Orders.CustomerID = Customers.CustomerID (Inner Join)

Step 1: Identify matching CustomerID values in Orders and Customers

Step 2: For each matching CustomerID, combine the row from Orders with the row from Customers

Step 3: Exclude Orders rows with CustomerID not in Customers (like C03)

Step 4: Final joined table columns: OrderID, CustomerID, OrderDate, CustomerName, Country

Cell Reference Map

Orders Table       Customers Table
+-------+---------+------------+   +------------+--------------+---------+
| A1    | B1      | C1         |   | E1         | F1           | G1      |
|OrderID|CustomerID|OrderDate  |   |CustomerID  |CustomerName  |Country  |
+-------+---------+------------+   +------------+--------------+---------+
| A2    | B2      | C2         |   | E2         | F2           | G2      |
|1001   | C01     | 2024-01-10 |   | C01        | Alice        | USA     |
| A3    | B3      | C3         |   | E3         | F3           | G3      |
|1002   | C02     | 2024-01-12 |   | C02        | Bob          | Canada  |
| A4    | B4      | C4         |   | E4         | F4           | G4      |
|1003   | C03     | 2024-01-15 |   | C04        | Diana        | UK      |
+-------+---------+------------+   +------------+--------------+---------+

Arrows: Orders.CustomerID (B2:B4) join to Customers.CustomerID (E2:E4)

Shows the two tables side by side with CustomerID columns highlighted as join keys.

Result

+---------+------------+------------+--------------+---------+
|OrderID  |CustomerID  |OrderDate   |CustomerName  |Country  |
+---------+------------+------------+--------------+---------+
|1001     |C01         |2024-01-10  |Alice         |USA      |
|1002     |C02         |2024-01-12  |Bob           |Canada   |
+---------+------------+------------+--------------+---------+

The joined table shows only orders with matching customers. Order 1003 is excluded because C03 is not in Customers.

Sheet Trace Quiz - 3 Questions

Which CustomerIDs appear in both Orders and Customers tables?

A C01 and C03

B C01 and C02

C C02 and C04

D C03 and C04

Key Result

Joining tables matches rows where key columns are equal and combines their columns into one table.

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