Cross-database joins in Tableau - Cell-by-Cell Formula Trace

Sample Data

Two tables from different databases: Orders (A1:C4) and Customers (E1:G4). Orders has order details, Customers has customer info.

Cell	Value
A1	OrderID
B1	CustomerID
C1	OrderAmount
A2	1001
B2	C001
C2	250
A3	1002
B3	C002
C3	450
A4	1003
B4	C003
C4	300
E1	CustomerID
F1	CustomerName
G1	Region
E2	C001
F2	Alice
G2	North
E3	C002
F3	Bob
G3	South
E4	C004
F4	Charlie
G4	East

Formula Trace

JOIN Orders.CustomerID = Customers.CustomerID

Step 1: Match Orders.CustomerID 'C001' with Customers.CustomerID

Step 2: Match Orders.CustomerID 'C002' with Customers.CustomerID

Step 3: Match Orders.CustomerID 'C003' with Customers.CustomerID

Step 4: Combine matched rows from Orders and Customers

Cell Reference Map

Orders Table       Customers Table
+-------+---------+------------+   +------------+--------------+--------+
|  A1   |   B1    |    C1      |   |    E1      |     F1       |   G1   |
|OrderID|CustomerID|OrderAmount|   |CustomerID  | CustomerName | Region |
+-------+---------+------------+   +------------+--------------+--------+
| 1001  |  C001   |    250     |   |   C001     |    Alice     | North  |
| 1002  |  C002   |    450     |   |   C002     |    Bob       | South  |
| 1003  |  C003   |    300     |   |   C004     |    Charlie   | East   |
+-------+---------+------------+   +------------+--------------+--------+

Arrows: Orders.CustomerID --> Customers.CustomerID

The join uses CustomerID from Orders (column B) and Customers (column E) to match rows across two different databases.

Result

+---------+------------+--------------+--------+
|OrderID  |OrderAmount |CustomerName  | Region |
+---------+------------+--------------+--------+
| 1001    | 250        | Alice        | North  |
| 1002    | 450        | Bob          | South  |
+---------+------------+--------------+--------+

Result of cross-database join showing orders with matching customer info. Order 1003 excluded because no matching customer.

Sheet Trace Quiz - 3 Questions

Which CustomerID from Orders does NOT find a match in Customers?

A C002

B C003

C C001

D C004

Key Result

Cross-database join matches key columns from two tables in different databases to combine related rows.

Practice

1. What is the main purpose of a cross-database join in Tableau?

easy

A. To export data to an external file

B. To create a copy of data within the same database

C. To combine tables from different data sources into one view

D. To filter data based on user input

Solution

1. Step 1: Understand cross-database join concept

   A cross-database join allows combining tables from different data sources in Tableau without moving data.

2. Step 2: Identify the main purpose

   The main goal is to create a richer view by joining data from multiple sources.

3. Final Answer:

   To combine tables from different data sources into one view -> Option C

4. Quick Check:

   Cross-database join = combine tables from different sources [OK]

Hint: Cross-database joins combine data from different sources [OK]

Common Mistakes:

• Thinking it copies data instead of joining
• Confusing with data filtering
• Assuming it exports data

2. Which of the following is the correct way to create a cross-database join in Tableau?

easy

A. Drag a table from one data source onto a table from another data source in the Data pane

B. Use the Data Extract option to combine tables

C. Create a calculated field to merge data sources

D. Export both tables and merge in Excel

Solution

1. Step 1: Recall Tableau's method for cross-database joins

   In Tableau, you drag a table from one data source onto another in the Data pane to create a cross-database join.

2. Step 2: Eliminate incorrect options

   Options A, B, and D describe other unrelated methods not used for cross-database joins in Tableau.

3. Final Answer:

   Drag a table from one data source onto a table from another data source in the Data pane -> Option A

4. Quick Check:

   Drag tables between sources = cross-database join [OK]

Hint: Drag tables between sources to join cross-database [OK]

Common Mistakes:

• Using data extracts instead of joins
• Trying to merge with calculated fields
• Exporting data outside Tableau

3. Given two tables from different databases joined on CustomerID, what happens if some CustomerID values exist only in one table?

medium

A. Those rows will be excluded from the join result

B. Those rows will appear with NULLs for missing columns

C. Tableau will throw an error and stop the join

D. Those rows will be duplicated in the result

Solution

1. Step 1: Understand join behavior with unmatched keys

   When joining tables, rows with keys only in one table appear with NULLs for columns from the other table if using a left or full outer join.

2. Step 2: Apply to cross-database join context

   In Tableau cross-database joins, unmatched keys show rows with NULLs, not excluded or duplicated.

3. Final Answer:

   Those rows will appear with NULLs for missing columns -> Option B

4. Quick Check:

   Unmatched keys = rows with NULLs [OK]

Hint: Unmatched keys show NULLs, not errors or duplicates [OK]

Common Mistakes:

• Assuming unmatched rows are excluded
• Expecting errors on unmatched keys
• Thinking unmatched rows duplicate

4. You created a cross-database join but Tableau shows incorrect results. Which of these is the most likely cause?

medium

A. You joined tables from the same database

B. You forgot to refresh the data extract

C. You used a calculated field instead of a join

D. Join keys have mismatched data types between sources

Solution

1. Step 1: Identify common issues in cross-database joins

   Mismatched data types in join keys cause incorrect or missing matches in joins.

2. Step 2: Evaluate other options

   Refreshing extracts or using calculated fields are unrelated to join key mismatches; joining same database tables is valid but not an error cause here.

3. Final Answer:

   Join keys have mismatched data types between sources -> Option D

4. Quick Check:

   Data type mismatch = wrong join results [OK]

Hint: Check join key data types match across sources [OK]

Common Mistakes:

• Ignoring data type differences
• Assuming refresh fixes join logic
• Confusing calculated fields with joins

5. You want to join a large SQL Server sales table with a small Excel customer list using a cross-database join. What is the best practice to optimize performance?

hard

A. Create extracts for both sources before joining

B. Use a left join with the Excel table on the left side

C. Import the Excel data into SQL Server and join there

D. Join directly without extracts to get live data

Solution

1. Step 1: Understand performance impact of cross-database joins

   Joining large and small tables across sources can slow performance; extracts improve speed by caching data.

2. Step 2: Evaluate options for optimization

   Creating extracts for both sources reduces query time and improves join speed compared to live joins.

3. Final Answer:

   Create extracts for both sources before joining -> Option A

4. Quick Check:

   Extracts speed up cross-database joins [OK]

Hint: Use extracts to speed up cross-database joins [OK]

Common Mistakes:

• Joining live without extracts on large data
• Importing Excel into SQL unnecessarily
• Placing smaller table on left without extracts