Joining on primary key to foreign key in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a primary key in a database table?

A primary key is a unique identifier for each row in a table. It ensures that no two rows have the same value in that column or set of columns.

beginner

What is a foreign key in a database table?

A foreign key is a column or set of columns in one table that refers to the primary key in another table. It creates a link between the two tables.

beginner

Why do we join tables on primary key to foreign key?

We join tables on primary key to foreign key to combine related data from two tables, like matching a customer with their orders.

beginner

Write a simple SQL query to join two tables on primary key to foreign key.

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

intermediate

What happens if you join tables on columns that are not primary key and foreign key?

The join might return incorrect or duplicate data because the link between tables is not guaranteed to be unique or valid.

What does a foreign key in a table represent?

A A link to the primary key in another table

B A unique identifier for each row in the same table

C A column that stores only numbers

D A column that cannot have NULL values

A foreign key links to the primary key in another table to connect related data.

Which SQL clause is used to combine rows from two tables based on a related column?

A WHERE

B GROUP BY

C JOIN

D ORDER BY

JOIN is used to combine rows from two tables based on a related column.

In the query: SELECT * FROM A JOIN B ON A.id = B.a_id; which is likely the primary key?

A B.a_id

B Neither is a primary key

C Both are primary keys

D A.id

A.id is likely the primary key, and B.a_id is the foreign key referencing it.

What is the main benefit of joining tables on primary key to foreign key?

A Faster data entry

B Ensures data integrity and correct matching

C Allows duplicate rows

D Removes all NULL values

Joining on primary key to foreign key ensures data integrity and correct matching of related rows.

If a foreign key value does not match any primary key, what happens in an INNER JOIN?

A The row is excluded from the result

B The row is duplicated

C The query fails with an error

D The row is included with NULLs

In an INNER JOIN, rows without matching keys are excluded from the result.

Explain in your own words why joining tables on primary key to foreign key is important.

Think about how two tables relate to each other in real life, like customers and their orders.

Describe how you would write a SQL query to join two tables using a primary key and foreign key.

Remember the format: SELECT * FROM Table1 JOIN Table2 ON Table1.key = Table2.key

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