Referential integrity enforcement in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What happens when deleting a parent row with ON DELETE CASCADE?

Given two tables departments and employees where employees.department_id references departments.id with ON DELETE CASCADE, what will be the result of deleting a department?

SQL

CREATE TABLE departments (id INT PRIMARY KEY, name VARCHAR(50));
CREATE TABLE employees (id INT PRIMARY KEY, name VARCHAR(50), department_id INT,
  FOREIGN KEY (department_id) REFERENCES departments(id) ON DELETE CASCADE);

INSERT INTO departments VALUES (1, 'HR'), (2, 'IT');
INSERT INTO employees VALUES (1, 'Alice', 1), (2, 'Bob', 2), (3, 'Charlie', 1);

DELETE FROM departments WHERE id = 1;

SELECT * FROM employees ORDER BY id;

A Only the department row is deleted; employees remain

B Delete fails with foreign key constraint error

C Rows with department_id = 1 are deleted; remaining employees: (2, 'Bob', 2)

D All employees remain unchanged

💡 Hint

Think about what ON DELETE CASCADE means for child rows.

The ON DELETE CASCADE option means that when a referenced row in the parent table is deleted, all child rows referencing it are automatically deleted. So deleting department 1 removes employees with department_id = 1.

📝 Syntax

intermediate

Which option correctly defines a foreign key with ON UPDATE SET NULL?

Choose the correct SQL statement to create a foreign key constraint that sets the child column to NULL when the parent key is updated.

A FOREIGN KEY (child_id) REFERENCES parent(id) ON UPDATE NO ACTION

B FOREIGN KEY (child_id) REFERENCES parent(id) ON UPDATE CASCADE

C FOREIGN KEY (child_id) REFERENCES parent(id) ON DELETE SET NULL

D FOREIGN KEY (child_id) REFERENCES parent(id) ON UPDATE SET NULL

💡 Hint

Focus on the ON UPDATE clause and the action SET NULL.

The ON UPDATE SET NULL clause means that if the parent key changes, the foreign key in the child row is set to NULL. Option D correctly uses this syntax.

🔧 Debug

advanced

Why does this foreign key constraint fail to create?

Given the following SQL, why does the foreign key constraint creation fail?

SQL

CREATE TABLE orders (
  order_id INT PRIMARY KEY,
  customer_id INT,
  FOREIGN KEY (customer_id) REFERENCES customers(id) ON DELETE RESTRICT
);

CREATE TABLE customers (
  id INT PRIMARY KEY,
  name VARCHAR(100)
);

A Primary key must be declared on <code>customer_id</code> in <code>orders</code>

B The referenced table <code>customers</code> must be created before <code>orders</code>

C The <code>ON DELETE RESTRICT</code> clause is invalid syntax

D The foreign key column <code>customer_id</code> must be declared NOT NULL

💡 Hint

Think about the order of table creation and foreign key references.

Foreign key constraints require the referenced table to exist before the referencing table is created. Here, orders references customers which is created later, causing failure.

❓ optimization

advanced

How to optimize foreign key checks for bulk inserts?

You need to insert thousands of rows into a child table with foreign keys. Which approach optimizes performance while maintaining referential integrity?

A Disable foreign key checks before insert, then re-enable after all inserts

B Insert rows one by one with foreign key checks enabled

C Drop foreign key constraints permanently before insert

D Insert rows in any order without foreign key constraints

💡 Hint

Consider temporarily disabling checks to speed up bulk operations.

Disabling foreign key checks temporarily allows faster bulk inserts. After inserts, re-enabling checks ensures integrity. Dropping constraints permanently risks data integrity.

🧠 Conceptual

expert

What is the effect of ON DELETE SET DEFAULT in referential integrity?

Consider a foreign key with ON DELETE SET DEFAULT. What happens when the referenced parent row is deleted?

A The child foreign key column is set to its default value

B The child row is deleted automatically

C The delete operation is rejected with an error

D The child foreign key column is set to NULL

💡 Hint

Think about what SET DEFAULT means for a foreign key column.

ON DELETE SET DEFAULT sets the foreign key column in the child row to its predefined default value when the referenced parent row is deleted.

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