Foreign key linking mental model in SQL - Mini Project: Build & Apply

Foreign Key Linking Mental Model

📖 Scenario: You are building a simple database for a library. The library has books and authors. Each book is written by one author. You want to link books to their authors using a foreign key.

🎯 Goal: Create two tables: authors and books. Then link the books table to the authors table using a foreign key on author_id. This will help you understand how foreign keys connect tables in a database.

📋 What You'll Learn

Create a table called authors with columns author_id (integer, primary key) and author_name (text).

Create a table called books with columns book_id (integer, primary key), title (text), and author_id (integer).

Add a foreign key constraint on books.author_id that references authors.author_id.

Use standard SQL syntax.

💡 Why This Matters

🌍 Real World

Foreign keys help keep data connected and consistent in real databases like libraries, stores, or social networks.

💼 Career

Understanding foreign keys is essential for database design and working with relational databases in many tech jobs.

1

Create the authors table

Write a SQL statement to create a table called authors with two columns: author_id as an integer primary key, and author_name as text.

SQL

-- Create the authors table with author_id and author_name
-- Your code here

Hint

Use CREATE TABLE authors and define author_id as INTEGER PRIMARY KEY.

2

Create the books table

Write a SQL statement to create a table called books with three columns: book_id as an integer primary key, title as text, and author_id as integer.

SQL

CREATE TABLE authors (
    author_id INTEGER PRIMARY KEY,
    author_name TEXT
);

-- Create the books table with book_id, title, and author_id
-- Your code here

Hint

Use CREATE TABLE books and define the columns as described.

3

Add the foreign key constraint

Modify the books table creation statement to add a foreign key constraint on author_id that references authors.author_id.

SQL

CREATE TABLE authors (
    author_id INTEGER PRIMARY KEY,
    author_name TEXT
);

-- Modify the books table to add a foreign key on author_id referencing authors.author_id
CREATE TABLE books (
    book_id INTEGER PRIMARY KEY,
    title TEXT,
    author_id INTEGER
    -- Your code here
);

Hint

Add FOREIGN KEY (author_id) REFERENCES authors(author_id) inside the books table definition.

4

Complete the foreign key linking

Ensure the full SQL code includes both authors and books tables with the foreign key constraint linking books.author_id to authors.author_id.

SQL

-- Verify the full SQL code for authors and books tables with foreign key
-- Your code here

Hint

Make sure both tables and the foreign key constraint are present in the code.

Practice

1. What is the main purpose of a foreign key in a database?

easy

A. To link one table to another and ensure data consistency

B. To store large amounts of text data

C. To speed up database queries

D. To create a backup of the database

Solution

1. Step 1: Understand the role of foreign keys

   A foreign key connects one table to another by referencing a primary key in the related table.

2. Step 2: Identify the purpose of this connection

   This connection helps keep data consistent and organized by preventing invalid data entries.

3. Final Answer:

   To link one table to another and ensure data consistency -> Option A

4. Quick Check:

   Foreign key = link tables + data consistency [OK]

Hint: Foreign keys link tables to keep data correct [OK]

Common Mistakes:

• Thinking foreign keys store data themselves
• Confusing foreign keys with indexes
• Believing foreign keys speed up queries directly

2. Which of the following is the correct syntax to declare a foreign key in SQL?

easy

A. FOREIGN KEY column_name REFERENCES other_table

B. PRIMARY KEY (column_name) REFERENCES other_table(other_column)

C. FOREIGN KEY (column_name) REFERENCES other_table(other_column)

D. KEY FOREIGN (column_name) REFERENCES other_table(other_column)

Solution

1. Step 1: Recall the standard foreign key syntax

   The correct syntax includes the keywords FOREIGN KEY, the column in parentheses, then REFERENCES followed by the referenced table and column in parentheses.

2. Step 2: Compare options to syntax

   FOREIGN KEY (column_name) REFERENCES other_table(other_column) matches the correct syntax exactly. Other options have wrong keyword order or missing parentheses.

3. Final Answer:

   FOREIGN KEY (column_name) REFERENCES other_table(other_column) -> Option C

4. Quick Check:

   FOREIGN KEY + REFERENCES + (table.column) = A [OK]

Hint: FOREIGN KEY (col) REFERENCES table(col) is correct syntax [OK]

Common Mistakes:

• Omitting parentheses around column names
• Swapping PRIMARY KEY with FOREIGN KEY
• Incorrect keyword order

3. Given these tables:
CREATE TABLE Authors (AuthorID INT PRIMARY KEY, Name VARCHAR(50));
CREATE TABLE Books (BookID INT PRIMARY KEY, Title VARCHAR(100), AuthorID INT, FOREIGN KEY (AuthorID) REFERENCES Authors(AuthorID));
What happens if you try to insert INSERT INTO Books (BookID, Title, AuthorID) VALUES (1, 'My Book', 99); when there is no author with AuthorID = 99 in Authors?

medium

A. The insert succeeds but AuthorID is set to NULL

B. The insert fails due to foreign key constraint violation

C. The database creates a new author with AuthorID 99 automatically

D. The insert succeeds and adds the book

Solution

1. Step 1: Understand foreign key constraint behavior

   A foreign key requires that the referenced value exists in the parent table to maintain data integrity.

2. Step 2: Apply this to the insert statement

   Since AuthorID 99 does not exist in Authors, the insert violates the foreign key constraint and fails.

3. Final Answer:

   The insert fails due to foreign key constraint violation -> Option B

4. Quick Check:

   Foreign key requires existing parent row = D [OK]

Hint: Foreign key insert fails if parent key missing [OK]

Common Mistakes:

• Assuming automatic creation of missing parent rows
• Thinking insert will succeed with NULL foreign key
• Ignoring foreign key constraints

4. Consider this table creation:
CREATE TABLE Orders (OrderID INT PRIMARY KEY, CustomerID INT, FOREIGN KEY CustomerID REFERENCES Customers(CustomerID));
What is wrong with this statement?

medium

A. Foreign key cannot reference Customers table

B. CustomerID should be declared as PRIMARY KEY

C. PRIMARY KEY must be declared after FOREIGN KEY

D. Missing parentheses around the foreign key column name

Solution

1. Step 1: Check foreign key syntax

   The foreign key column name must be enclosed in parentheses after FOREIGN KEY.

2. Step 2: Identify the error in the statement

   The statement uses FOREIGN KEY CustomerID without parentheses, which is invalid syntax.

3. Final Answer:

   Missing parentheses around the foreign key column name -> Option D

4. Quick Check:

   FOREIGN KEY (col) needs parentheses [OK]

Hint: Always use parentheses around foreign key columns [OK]

Common Mistakes:

• Omitting parentheses in FOREIGN KEY declaration
• Misordering PRIMARY and FOREIGN KEY declarations
• Confusing foreign key with primary key requirements

5. You have two tables:
CREATE TABLE Departments (DeptID INT PRIMARY KEY, DeptName VARCHAR(50));
CREATE TABLE Employees (EmpID INT PRIMARY KEY, EmpName VARCHAR(50), DeptID INT, FOREIGN KEY (DeptID) REFERENCES Departments(DeptID) ON DELETE SET NULL);
If a department is deleted, what happens to employees linked to that department?

hard

A. Their DeptID is set to NULL automatically

B. The delete is blocked and fails

C. Employees linked to that department are deleted

D. Nothing happens; DeptID remains unchanged

Solution

1. Step 1: Understand ON DELETE SET NULL behavior

   This option means when the referenced row is deleted, the foreign key column in dependent rows is set to NULL.

2. Step 2: Apply to Employees and Departments

   Deleting a department sets DeptID to NULL in Employees who referenced it, keeping employees but removing the link.

3. Final Answer:

   Their DeptID is set to NULL automatically -> Option A

4. Quick Check:

   ON DELETE SET NULL means foreign keys become NULL [OK]

Hint: ON DELETE SET NULL clears foreign keys on delete [OK]

Common Mistakes:

• Assuming delete blocks or cascades employees
• Thinking employees get deleted automatically
• Ignoring ON DELETE action effects