Referential integrity enforcement in SQL - Mini Project: Build & Apply

Referential Integrity Enforcement in SQL

📖 Scenario: You are managing a small library database. You need to keep track of books and their authors. Each book must be linked to an author to ensure data consistency.

🎯 Goal: Create two tables, Authors and Books, and enforce referential integrity so that every book references a valid author.

📋 What You'll Learn

Create an Authors table with columns AuthorID (primary key) and Name.

Create a Books table with columns BookID (primary key), Title, and AuthorID.

Add a foreign key constraint on Books.AuthorID referencing Authors.AuthorID.

Ensure that deleting an author will delete all their books automatically.

💡 Why This Matters

🌍 Real World

Libraries, bookstores, and many applications use referential integrity to keep related data accurate and consistent.

💼 Career

Database administrators and developers must enforce referential integrity to prevent orphan records and maintain data quality.

1

Create the Authors table

Write a SQL statement to create a table called Authors with two columns: AuthorID as an integer primary key, and Name as a text field.

SQL

-- Your code here to create the Authors table

Hint

Use CREATE TABLE with PRIMARY KEY on AuthorID.

2

Create the Books table

Write a SQL statement to create a table called Books with three columns: BookID as an integer primary key, Title as a text field, and AuthorID as an integer.

SQL

CREATE TABLE Authors (
    AuthorID INT PRIMARY KEY,
    Name TEXT
);
-- Your code here to create the Books table

Hint

Define BookID as primary key and include AuthorID as an integer column.

3

Add foreign key constraint

Modify the Books table creation to add a foreign key constraint on AuthorID that references Authors.AuthorID.

SQL

CREATE TABLE Authors (
    AuthorID INT PRIMARY KEY,
    Name TEXT
);

CREATE TABLE Books (
    BookID INT PRIMARY KEY,
    Title TEXT,
    AuthorID INT,
    -- Your code here to add the foreign key constraint
);

Hint

Use FOREIGN KEY (AuthorID) REFERENCES Authors(AuthorID) inside the Books table definition.

4

Enforce cascading delete

Update the foreign key constraint on Books.AuthorID to include ON DELETE CASCADE so that deleting an author removes their books automatically.

SQL

CREATE TABLE Authors (
    AuthorID INT PRIMARY KEY,
    Name TEXT
);

CREATE TABLE Books (
    BookID INT PRIMARY KEY,
    Title TEXT,
    AuthorID INT,
    FOREIGN KEY (AuthorID) REFERENCES Authors(AuthorID) -- Add cascading delete here
);

Hint

Add ON DELETE CASCADE after the foreign key reference.

Practice

1. What is the main purpose of referential integrity in a database?

easy

A. To speed up query execution

B. To ensure relationships between tables remain consistent

C. To store large amounts of data efficiently

D. To create backup copies of the database

Solution

1. Step 1: Understand referential integrity concept

   Referential integrity ensures that foreign keys in one table correctly reference existing rows in another table.

2. Step 2: Identify the main purpose

   This prevents orphan records and keeps data relationships consistent and safe.

3. Final Answer:

   To ensure relationships between tables remain consistent -> Option B

4. Quick Check:

   Referential integrity = consistent relationships [OK]

Hint: Referential integrity means keeping table links correct [OK]

Common Mistakes:

• Confusing referential integrity with performance optimization
• Thinking it creates backups
• Assuming it stores data efficiently

2. Which SQL statement correctly defines a foreign key with referential integrity enforcement?

easy

A. INSERT INTO Orders (OrderID, CustomerID) VALUES (1, 100);

B. CREATE TABLE Orders (OrderID INT, CustomerID INT PRIMARY KEY);

C. ALTER TABLE Orders ADD FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID);

D. SELECT * FROM Orders WHERE CustomerID = Customers.CustomerID;

Solution

1. Step 1: Identify foreign key syntax

   The correct syntax to add a foreign key is using ALTER TABLE with ADD FOREIGN KEY referencing another table's column.

2. Step 2: Check each option

   ALTER TABLE Orders ADD FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID); correctly adds a foreign key constraint. CREATE TABLE Orders (OrderID INT, CustomerID INT PRIMARY KEY); wrongly sets CustomerID as primary key without foreign key. INSERT INTO Orders (OrderID, CustomerID) VALUES (1, 100); is an insert, not a constraint. SELECT * FROM Orders WHERE CustomerID = Customers.CustomerID; is a select query, not a constraint definition.

3. Final Answer:

   ALTER TABLE Orders ADD FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID); -> Option C

4. Quick Check:

   Foreign key syntax = ALTER TABLE Orders ADD FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID); [OK]

Hint: Foreign keys use ALTER TABLE ADD FOREIGN KEY ... REFERENCES [OK]

Common Mistakes:

• Confusing primary key with foreign key syntax
• Using INSERT or SELECT instead of constraint definition
• Missing REFERENCES keyword

3. Given these tables:

CREATE TABLE Customers (CustomerID INT PRIMARY KEY, Name VARCHAR(50));
CREATE TABLE Orders (OrderID INT PRIMARY KEY, CustomerID INT, FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID) ON DELETE CASCADE);

What happens if a customer with CustomerID = 5 is deleted?

medium

A. All orders with CustomerID = 5 are also deleted

B. The delete fails due to foreign key constraint

C. Orders with CustomerID = 5 remain unchanged

D. CustomerID in Orders is set to NULL for those orders

Solution

1. Step 1: Understand ON DELETE CASCADE

   The ON DELETE CASCADE option means deleting a referenced row causes all related rows to be deleted automatically.

2. Step 2: Apply to the scenario

   Deleting customer with CustomerID=5 will delete all orders linked to that customer in Orders table.

3. Final Answer:

   All orders with CustomerID = 5 are also deleted -> Option A

4. Quick Check:

   ON DELETE CASCADE = delete related rows [OK]

Hint: ON DELETE CASCADE deletes related rows automatically [OK]

Common Mistakes:

• Thinking delete will fail due to constraint
• Assuming related rows remain unchanged
• Confusing CASCADE with SET NULL

4. You have this foreign key constraint:

FOREIGN KEY (ProductID) REFERENCES Products(ProductID) ON DELETE SET NULL

Which error will occur if you try to delete a product that is referenced by an order, but the ProductID column in Orders is defined as NOT NULL?

medium

A. Delete fails due to NOT NULL constraint violation

B. Delete succeeds and sets ProductID to NULL

C. Delete succeeds and removes the order row

D. Delete succeeds without affecting Orders

Solution

1. Step 1: Understand ON DELETE SET NULL behavior

   This option sets the foreign key column to NULL in referencing rows when the referenced row is deleted.

2. Step 2: Check NOT NULL constraint conflict

   If the foreign key column is NOT NULL, setting it to NULL violates the column constraint, causing the delete to fail.

3. Final Answer:

   Delete fails due to NOT NULL constraint violation -> Option A

4. Quick Check:

   SET NULL + NOT NULL column = delete fails [OK]

Hint: SET NULL fails if foreign key column is NOT NULL [OK]

Common Mistakes:

• Assuming delete succeeds and sets NULL anyway
• Thinking delete removes referencing rows
• Ignoring NOT NULL constraint on foreign key

5. You want to enforce referential integrity between Employees and Departments tables. When a department is deleted, you want all employees in that department to be reassigned to department ID 0 (which means 'Unassigned'). Which foreign key option should you use?

hard

A. FOREIGN KEY (DepartmentID) REFERENCES Departments(DepartmentID) ON DELETE RESTRICT

B. FOREIGN KEY (DepartmentID) REFERENCES Departments(DepartmentID) ON DELETE CASCADE

C. FOREIGN KEY (DepartmentID) REFERENCES Departments(DepartmentID) ON DELETE SET NULL

D. FOREIGN KEY (DepartmentID) REFERENCES Departments(DepartmentID) ON DELETE SET DEFAULT

Solution

1. Step 1: Understand ON DELETE SET DEFAULT

   This option sets the foreign key column to its default value when the referenced row is deleted.

2. Step 2: Match requirement

   Since you want employees reassigned to department ID 0, set DepartmentID column default to 0 and use ON DELETE SET DEFAULT to assign that value automatically.

3. Final Answer:

   FOREIGN KEY (DepartmentID) REFERENCES Departments(DepartmentID) ON DELETE SET DEFAULT -> Option D

4. Quick Check:

   Reassign on delete = ON DELETE SET DEFAULT [OK]

Hint: Use ON DELETE SET DEFAULT to assign default on delete [OK]

Common Mistakes:

• Using CASCADE deletes employees instead of reassigning
• Using SET NULL when column disallows NULL
• Using RESTRICT blocks deletion