Joining on primary key to foreign key in SQL - Step-by-Step Execution

Concept Flow - Joining on primary key to foreign key

Start with two tables

↓

Identify primary key in Table A

↓

Identify foreign key in Table B

↓

Match rows where TableA.primary_key = TableB.foreign_key

↓

Combine matched rows into one result

↓

Output joined table with columns from both tables

We start with two tables, find the primary key in one and the matching foreign key in the other, then combine rows where these keys match to create a joined result.

Execution Sample

SQL

SELECT Orders.OrderID, Customers.CustomerName
FROM Orders
JOIN Customers ON Orders.CustomerID = Customers.CustomerID;

This query joins Orders and Customers tables by matching Orders.CustomerID (foreign key) with Customers.CustomerID (primary key) to list orders with customer names.

Execution Table

Step	Action	Orders Row	Customers Row	Join Condition	Result Row
1	Check first Orders row	OrderID=101, CustomerID=1	CustomerID=1, CustomerName='Alice'	1 = 1 (True)	OrderID=101, CustomerName='Alice'
2	Check second Orders row	OrderID=102, CustomerID=2	CustomerID=1, CustomerName='Alice'	2 = 1 (False)	No row
3	Check second Orders row	OrderID=102, CustomerID=2	CustomerID=2, CustomerName='Bob'	2 = 2 (True)	OrderID=102, CustomerName='Bob'
4	Check third Orders row	OrderID=103, CustomerID=3	CustomerID=1, CustomerName='Alice'	3 = 1 (False)	No row
5	Check third Orders row	OrderID=103, CustomerID=3	CustomerID=2, CustomerName='Bob'	3 = 2 (False)	No row
6	Check third Orders row	OrderID=103, CustomerID=3	CustomerID=3, CustomerName='Charlie'	3 = 3 (True)	OrderID=103, CustomerName='Charlie'
7	No more Orders rows				End of join

💡 All Orders rows checked against Customers rows; join completed.

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 6	Final
Current Orders Row	None	OrderID=101, CustomerID=1	OrderID=102, CustomerID=2	OrderID=103, CustomerID=3	None (end)
Current Customers Row	None	CustomerID=1, CustomerName='Alice'	CustomerID=2, CustomerName='Bob'	CustomerID=3, CustomerName='Charlie'	None (end)
Result Rows	Empty	[(101, 'Alice')]	[(101, 'Alice'), (102, 'Bob')]	[(101, 'Alice'), (102, 'Bob'), (103, 'Charlie')]	Final result set

Key Moments - 3 Insights

Why do we compare Orders.CustomerID to Customers.CustomerID?

Because Orders.CustomerID is a foreign key that references Customers.CustomerID, the primary key. The join matches rows where these keys are equal, as shown in execution_table steps 1, 3, and 6.

Why do some Orders rows check multiple Customers rows?

Because the join tries to find a matching Customers row for each Orders row. If the first Customers row doesn't match (false condition), it checks the next one until it finds a match or runs out of rows, as seen in steps 2, 4, and 5.

What happens if no matching Customers row is found?

No joined row is produced for that Orders row. The execution_table shows 'No row' in the Result Row column when the join condition is false.

Visual Quiz - 3 Questions

Look at the execution table, what is the Result Row at Step 3?

A OrderID=102, CustomerName='Alice'

B OrderID=102, CustomerName='Bob'

C No row

D OrderID=101, CustomerName='Alice'

💡 Hint

Check the Result Row column in execution_table at Step 3.

At which step does the join find the match for OrderID=103?

A Step 4

B Step 5

C Step 6

D Step 7

💡 Hint

Look for OrderID=103 in Orders Row and a true join condition in execution_table.

If Orders had a row with CustomerID=4, what would happen in the join?

A It would produce no joined row because no matching Customers row exists

B It would join with Customers.CustomerID=4 row

C It would cause an error

D It would join with the first Customers row

💡 Hint

Refer to key_moments about what happens when no matching Customers row is found.

Concept Snapshot

JOINING ON PRIMARY KEY TO FOREIGN KEY

- Use JOIN with ON clause: TableA.primary_key = TableB.foreign_key
- Matches rows where keys are equal
- Combines columns from both tables in result
- Rows without matches are excluded in INNER JOIN
- Common for linking related data across tables

Full Transcript

This visual execution shows how SQL joins two tables by matching a primary key in one table with a foreign key in another. We start with Orders and Customers tables. For each Orders row, we check Customers rows to find where Orders.CustomerID equals Customers.CustomerID. When they match, we combine the rows into one result row with order ID and customer name. Steps 1, 3, and 6 show successful matches. Steps 2, 4, and 5 show checks that fail and produce no result. The process continues until all Orders rows are checked. This method links related data across tables using keys, producing a combined view of information.

Practice

1. What is the main purpose of joining tables on a primary key to a foreign key in SQL?

easy

A. To create a new table with all columns from both tables without conditions

B. To delete duplicate rows from a table

C. To combine related data from two tables based on a unique identifier

D. To update values in one table using values from another unrelated table

Solution

1. Step 1: Understand primary and foreign keys

   A primary key uniquely identifies each record in a table, and a foreign key points to that primary key in another table.

2. Step 2: Purpose of joining on these keys

   Joining on primary key to foreign key connects related records from two tables, combining their data meaningfully.

3. Final Answer:

   To combine related data from two tables based on a unique identifier -> Option C

4. Quick Check:

   Join on primary to foreign key = combine related data [OK]

Hint: Primary key links uniquely; join combines related rows [OK]

Common Mistakes:

• Thinking join deletes duplicates
• Assuming join creates unrelated combinations
• Confusing join with update or delete operations

2. Which of the following SQL JOIN statements correctly joins table Orders with Customers on the primary key CustomerID and foreign key CustomerID?

easy

A. SELECT * FROM Orders JOIN Customers ON Orders.OrderID = Customers.CustomerID;

B. SELECT * FROM Orders JOIN Customers ON Orders.OrderDate = Customers.CustomerID;

C. SELECT * FROM Orders JOIN Customers ON Orders.CustomerID = Customers.OrderID;

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

Solution

1. Step 1: Identify correct keys for join

   The primary key in Customers is CustomerID, and Orders has CustomerID as foreign key.

2. Step 2: Match keys in JOIN condition

   The join must be ON Orders.CustomerID = Customers.CustomerID to link related records.

3. Final Answer:

   SELECT * FROM Orders JOIN Customers ON Orders.CustomerID = Customers.CustomerID; -> Option D

4. Quick Check:

   Join on matching CustomerID keys = SELECT * FROM Orders JOIN Customers ON Orders.CustomerID = Customers.CustomerID; [OK]

Hint: Join ON foreign key = primary key column names [OK]

Common Mistakes:

• Mixing up primary and foreign key columns
• Joining on unrelated columns like OrderDate
• Using wrong table columns in ON clause

3. Given tables Employees (primary key EmployeeID) and Departments (foreign key ManagerID referencing EmployeeID), what will this query return?

SELECT Employees.Name, Departments.DepartmentName FROM Employees JOIN Departments ON Employees.EmployeeID = Departments.ManagerID;

medium

A. Syntax error due to wrong join condition

B. List of employee names who manage departments with their department names

C. List of departments without any employee names

D. List of all employees with all departments regardless of manager

Solution

1. Step 1: Understand join condition

   The join matches Employees.EmployeeID to Departments.ManagerID, linking managers to their departments.

2. Step 2: Result of the join

   The query returns names of employees who are managers and the names of the departments they manage.

3. Final Answer:

   List of employee names who manage departments with their department names -> Option B

4. Quick Check:

   Join on manager ID returns managers with departments [OK]

Hint: Join foreign key to primary key shows related records [OK]

Common Mistakes:

• Thinking it returns all employees regardless of management
• Assuming syntax error due to join condition
• Expecting departments without managers

4. Consider these tables:
Products(ProductID PK, Name)
Sales(ProductID FK, Quantity)
Why does this query cause an error?

SELECT * FROM Products JOIN Sales ON Products.ID = Sales.ProductID;

medium

A. Column Products.ID does not exist, causing an error

B. Foreign key cannot be used in JOIN condition

C. JOIN syntax is incorrect, missing JOIN type

D. Sales table must be listed first in FROM clause

Solution

1. Step 1: Check column names in JOIN condition

   The Products table has ProductID as primary key, not ID.

2. Step 2: Identify cause of error

   Using Products.ID causes an error because that column does not exist.

3. Final Answer:

   Column Products.ID does not exist, causing an error -> Option A

4. Quick Check:

   Wrong column name in JOIN = error [OK]

Hint: Verify column names exactly before joining [OK]

Common Mistakes:

• Using wrong or misspelled column names
• Thinking foreign keys can't be joined
• Assuming JOIN type is mandatory

5. You have two tables:
Authors(AuthorID PK, Name)
Books(BookID PK, Title, AuthorID FK)
Write a query to list each author with the count of books they wrote, including authors with zero books.

hard

A. SELECT Authors.Name, COUNT(Books.BookID) FROM Authors LEFT JOIN Books ON Authors.AuthorID = Books.AuthorID GROUP BY Authors.Name;

B. SELECT Authors.Name, COUNT(Books.BookID) FROM Authors JOIN Books ON Authors.AuthorID = Books.AuthorID GROUP BY Authors.Name;

C. SELECT Authors.Name, COUNT(*) FROM Books JOIN Authors ON Books.AuthorID = Authors.AuthorID GROUP BY Authors.Name;

D. SELECT Authors.Name, COUNT(Books.BookID) FROM Books LEFT JOIN Authors ON Books.AuthorID = Authors.AuthorID GROUP BY Authors.Name;

Solution

1. Step 1: Use LEFT JOIN to include all authors

   LEFT JOIN keeps all authors even if they have no matching books.

2. Step 2: Count books per author

   COUNT(Books.BookID) counts books; NULLs for authors without books count as zero.

3. Final Answer:

   SELECT Authors.Name, COUNT(Books.BookID) FROM Authors LEFT JOIN Books ON Authors.AuthorID = Books.AuthorID GROUP BY Authors.Name; -> Option A

4. Quick Check:

   LEFT JOIN + COUNT on foreign key = authors with book counts [OK]

Hint: Use LEFT JOIN to include all from primary key table [OK]

Common Mistakes:

• Using INNER JOIN excludes authors with zero books
• Counting * instead of foreign key column
• Joining tables in wrong order