Foreign key ON DELETE behavior in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Effect of ON DELETE CASCADE on child table rows

Consider two tables: authors and books. The books table has a foreign key author_id referencing authors(id) with ON DELETE CASCADE. If an author with id = 5 is deleted, what happens to the books written by that author?

SQL

CREATE TABLE authors (id INT PRIMARY KEY, name VARCHAR(100));
CREATE TABLE books (id INT PRIMARY KEY, title VARCHAR(100), author_id INT, FOREIGN KEY (author_id) REFERENCES authors(id) ON DELETE CASCADE);

INSERT INTO authors VALUES (5, 'Jane Doe');
INSERT INTO books VALUES (101, 'Book A', 5), (102, 'Book B', 5);

DELETE FROM authors WHERE id = 5;

SELECT * FROM books WHERE author_id = 5;

A Books with author_id = 5 remain unchanged.

B No rows in books with author_id = 5 remain; they are deleted.

C Books with author_id = 5 have author_id set to NULL.

D Deletion of author with id 5 fails due to foreign key constraint.

💡 Hint

Think about what ON DELETE CASCADE means for dependent rows.

The ON DELETE CASCADE option means that when a referenced row in the parent table (authors) is deleted, all rows in the child table (books) that reference it are automatically deleted as well.

❓ query_result

intermediate

Behavior of ON DELETE SET NULL on foreign key

Given a orders table with a foreign key customer_id referencing customers(id) with ON DELETE SET NULL, what is the result of deleting a customer with id = 10?

SQL

CREATE TABLE customers (id INT PRIMARY KEY, name VARCHAR(100));
CREATE TABLE orders (id INT PRIMARY KEY, order_date DATE, customer_id INT, FOREIGN KEY (customer_id) REFERENCES customers(id) ON DELETE SET NULL);

INSERT INTO customers VALUES (10, 'Alice');
INSERT INTO orders VALUES (201, '2024-01-01', 10), (202, '2024-01-02', 10);

DELETE FROM customers WHERE id = 10;

SELECT * FROM orders WHERE id IN (201, 202);

A The orders with customer_id 10 remain, but their customer_id is set to NULL.

B The orders with customer_id 10 are deleted.

C The deletion of customer with id 10 fails due to foreign key constraint.

D The orders with customer_id 10 remain unchanged.

💡 Hint

Consider what ON DELETE SET NULL does to foreign key columns.

With ON DELETE SET NULL, when the referenced parent row is deleted, the foreign key column in the child rows is set to NULL instead of deleting the child rows.

🧠 Conceptual

advanced

Understanding ON DELETE RESTRICT behavior

What happens if you try to delete a row from a parent table that has child rows referencing it, and the foreign key is defined with ON DELETE RESTRICT?

A The parent row is deleted and child rows are deleted automatically.

B The parent row is deleted but child rows remain with invalid foreign keys.

C The parent row is deleted and child rows have their foreign key set to NULL.

D The deletion of the parent row is blocked and raises an error.

💡 Hint

Think about what RESTRICT means in terms of deletion.

ON DELETE RESTRICT prevents deletion of a parent row if any child rows reference it. The database raises an error to maintain referential integrity.

📝 Syntax

advanced

Correct syntax for foreign key with ON DELETE SET DEFAULT

Which of the following SQL statements correctly creates a foreign key with ON DELETE SET DEFAULT behavior?

A ALTER TABLE orders ADD FOREIGN KEY (customer_id) REFERENCES customers(id) ON DELETE SET DEFAULT;

B ALTER TABLE orders ADD CONSTRAINT fk_customer FOREIGN KEY (customer_id) REFERENCES customers(id) ON DELETE SET NULL;

C ALTER TABLE orders ADD CONSTRAINT fk_customer FOREIGN KEY (customer_id) REFERENCES customers(id) ON DELETE SET DEFAULT;

D ALTER TABLE orders ADD CONSTRAINT fk_customer FOREIGN KEY (customer_id) REFERENCES customers(id) ON DELETE CASCADE;

💡 Hint

Check the full syntax for adding a named foreign key constraint with ON DELETE behavior.

The correct syntax requires naming the constraint and specifying the foreign key columns and referenced table, followed by the ON DELETE SET DEFAULT clause.

🔧 Debug

expert

Diagnosing foreign key deletion error

You have two tables: departments and employees. The employees table has a foreign key department_id referencing departments(id) with ON DELETE NO ACTION. When you try to delete a department that has employees, you get an error. What is the cause?

SQL

CREATE TABLE departments (id INT PRIMARY KEY, name VARCHAR(100));
CREATE TABLE employees (id INT PRIMARY KEY, name VARCHAR(100), department_id INT, FOREIGN KEY (department_id) REFERENCES departments(id) ON DELETE NO ACTION);

INSERT INTO departments VALUES (1, 'HR');
INSERT INTO employees VALUES (1001, 'John', 1);

DELETE FROM departments WHERE id = 1;

A The foreign key constraint with ON DELETE NO ACTION prevents deleting a department if employees reference it.

B The foreign key constraint is missing, so deletion fails.

C The employees table has no rows, so deletion should succeed.

D The ON DELETE NO ACTION allows deletion but sets department_id to NULL.

💡 Hint

Consider what ON DELETE NO ACTION means for deletion of referenced rows.

ON DELETE NO ACTION means the database will prevent deletion of a parent row if child rows exist referencing it, raising an error to maintain integrity.

Practice

1. What does the ON DELETE CASCADE option do in a foreign key constraint?

easy

A. Deletes all related child rows automatically when the parent row is deleted.

B. Prevents deletion of the parent row if related child rows exist.

C. Sets the foreign key in child rows to NULL when the parent row is deleted.

D. Does nothing to child rows when the parent row is deleted.

Solution

1. Step 1: Understand ON DELETE CASCADE behavior

   This option means that when a parent row is deleted, all child rows linked by the foreign key are also deleted automatically.

2. Step 2: Compare with other options

   Other options like RESTRICT block deletion, and SET NULL clears the foreign key, so only CASCADE deletes child rows.

3. Final Answer:

   Deletes all related child rows automatically when the parent row is deleted. -> Option A

4. Quick Check:

   ON DELETE CASCADE = delete related rows [OK]

Hint: CASCADE means delete children when parent is deleted [OK]

Common Mistakes:

• Confusing CASCADE with RESTRICT
• Thinking CASCADE sets foreign keys to NULL
• Assuming CASCADE does nothing

2. Which of the following is the correct syntax to add a foreign key with ON DELETE SET NULL in SQL?

easy

A. FOREIGN KEY (child_id) REFERENCES parent(id) ON DELETE CASCADE

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

C. FOREIGN KEY (child_id) REFERENCES parent(id) ON DELETE RESTRICT

D. FOREIGN KEY (child_id) REFERENCES parent(id) ON DELETE NO ACTION

Solution

1. Step 1: Identify the syntax for ON DELETE SET NULL

   The correct syntax includes the phrase ON DELETE SET NULL after the foreign key reference.

2. Step 2: Match options with syntax

   FOREIGN KEY (child_id) REFERENCES parent(id) ON DELETE SET NULL exactly matches the required syntax for setting foreign keys to NULL on parent deletion.

3. Final Answer:

   FOREIGN KEY (child_id) REFERENCES parent(id) ON DELETE SET NULL -> Option B

4. Quick Check:

   ON DELETE SET NULL syntax = FOREIGN KEY (child_id) REFERENCES parent(id) ON DELETE SET NULL [OK]

Hint: Look for 'ON DELETE SET NULL' phrase exactly [OK]

Common Mistakes:

• Choosing CASCADE instead of SET NULL
• Confusing RESTRICT and NO ACTION
• Missing ON DELETE clause

3. Given these tables:

CREATE TABLE parent (id INT PRIMARY KEY);
CREATE TABLE child (id INT PRIMARY KEY, parent_id INT, FOREIGN KEY (parent_id) REFERENCES parent(id) ON DELETE RESTRICT);
INSERT INTO parent VALUES (1);
INSERT INTO child VALUES (10, 1);

What happens if you run DELETE FROM parent WHERE id = 1;?

medium

A. The parent row is deleted, and the child row is also deleted.

B. The parent row is deleted, but the child row's parent_id is set to NULL.

C. The delete succeeds and leaves the child row with a dangling parent_id.

D. The delete fails because the child row exists and ON DELETE RESTRICT blocks it.

Solution

1. Step 1: Understand ON DELETE RESTRICT effect

   RESTRICT prevents deleting a parent row if any child rows reference it.

2. Step 2: Apply to given data

   Since child row with parent_id=1 exists, deleting parent id=1 is blocked.

3. Final Answer:

   The delete fails because the child row exists and ON DELETE RESTRICT blocks it. -> Option D

4. Quick Check:

   ON DELETE RESTRICT blocks delete if children exist [OK]

Hint: RESTRICT blocks delete if child rows exist [OK]

Common Mistakes:

• Assuming CASCADE behavior with RESTRICT
• Thinking child foreign key becomes NULL
• Believing delete always succeeds

4. You wrote this SQL:

ALTER TABLE child ADD CONSTRAINT fk_parent FOREIGN KEY (parent_id) REFERENCES parent(id) ON DELETE SET NULL;

But when you delete a parent row, the child row's parent_id does not become NULL. What is the likely problem?

medium

A. The foreign key syntax is incorrect and ignored by the database.

B. ON DELETE SET NULL only works with CASCADE.

C. The parent_id column is NOT NULL, so it cannot be set to NULL.

D. You must delete child rows manually before deleting parent.

Solution

1. Step 1: Check column nullability

   ON DELETE SET NULL requires the foreign key column to allow NULL values.

2. Step 2: Identify the cause

   If parent_id is NOT NULL, the database cannot set it to NULL, so it leaves it unchanged.

3. Final Answer:

   The parent_id column is NOT NULL, so it cannot be set to NULL. -> Option C

4. Quick Check:

   SET NULL needs nullable foreign key column [OK]

Hint: Foreign key must allow NULL for SET NULL to work [OK]

Common Mistakes:

• Assuming syntax error causes issue
• Thinking SET NULL requires CASCADE
• Believing manual delete is always needed

5. You have two tables:

CREATE TABLE orders (order_id INT PRIMARY KEY);
CREATE TABLE order_items (item_id INT PRIMARY KEY, order_id INT, FOREIGN KEY (order_id) REFERENCES orders(order_id) ON DELETE CASCADE);

You want to delete an order and all its items safely. However, sometimes order_items.order_id can be NULL for items not linked to any order. What is the best ON DELETE behavior to use for the foreign key to avoid errors and keep data consistent?

hard

A. ON DELETE CASCADE, because it deletes all linked items automatically.

B. ON DELETE NO ACTION, to do nothing on delete.

C. ON DELETE RESTRICT, to prevent deleting orders with linked items.

D. ON DELETE SET NULL, so deleting an order sets linked items' order_id to NULL.

Solution

1. Step 1: Identify the goal

   You want to delete an order and automatically delete all its related items safely.

2. Step 2: Evaluate ON DELETE CASCADE

   CASCADE deletes all child rows (order_items) where order_id matches the deleted order, achieving the goal automatically.

3. Step 3: Handle NULL values

   Items with NULL order_id are not referencing any order, so CASCADE leaves them untouched, maintaining consistency without errors.

4. Final Answer:

   ON DELETE CASCADE, because it deletes all linked items automatically. -> Option A

5. Quick Check:

   Delete order + linked items safely = ON DELETE CASCADE [OK]

Hint: CASCADE deletes linked items automatically, ignores NULLs [OK]

Common Mistakes:

• Choosing SET NULL, which orphans items instead of deleting
• Using RESTRICT, blocking necessary deletes
• Picking NO ACTION, requiring manual item deletes