Foreign key ON DELETE behavior in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

It automatically deletes all rows in the child table that reference the deleted row in the parent table. This keeps data consistent by removing dependent records.

beginner

Explain the ON DELETE SET NULL behavior in foreign keys.

When a referenced row in the parent table is deleted, the foreign key column in the child table is set to NULL. This means the child row no longer points to a parent row.

intermediate

What happens if a foreign key has ON DELETE RESTRICT and you try to delete a parent row?

The deletion is blocked if there are any child rows referencing the parent. You must first remove or update child rows before deleting the parent.

advanced

Describe the ON DELETE NO ACTION behavior.

It prevents deletion of a parent row if child rows exist, similar to RESTRICT. The difference is in timing: NO ACTION checks constraints at the end of the statement or transaction.

beginner

What is the default behavior if ON DELETE is not specified in a foreign key?

The default is usually NO ACTION, meaning the database will prevent deleting a parent row if child rows exist referencing it.

What does ON DELETE CASCADE do when a parent row is deleted?

A Deletes all child rows referencing the parent row

B Sets child foreign keys to NULL

C Prevents deletion of the parent row

D Does nothing automatically

ON DELETE CASCADE deletes all child rows that reference the deleted parent row to keep data consistent.

Which ON DELETE option sets the foreign key in child rows to NULL when the parent is deleted?

A CASCADE

B RESTRICT

C SET NULL

D NO ACTION

SET NULL sets the foreign key column in child rows to NULL when the referenced parent row is deleted.

If a foreign key uses ON DELETE RESTRICT, what happens when you try to delete a parent row with children?

A Deletion is blocked until children are removed

B Parent row is deleted and children updated

C Children are deleted automatically

D Foreign keys are set to NULL

RESTRICT blocks deletion of the parent row if child rows exist referencing it.

What is the main difference between NO ACTION and RESTRICT in ON DELETE behavior?

A NO ACTION deletes children, RESTRICT does not

B They behave exactly the same

C NO ACTION sets foreign keys to NULL, RESTRICT blocks deletion

D RESTRICT checks constraints immediately, NO ACTION checks at statement end

RESTRICT checks constraints immediately, while NO ACTION defers constraint checking until the end of the statement or transaction.

What is the default ON DELETE behavior if none is specified?

A SET NULL

B NO ACTION

C CASCADE

D RESTRICT

The default ON DELETE behavior is usually NO ACTION, which prevents deletion if child rows exist.

Describe the different ON DELETE behaviors available for foreign keys and when you might use each.

Think about how deleting a parent row affects related child rows.

Explain why ON DELETE CASCADE can be useful and what risks it might have.

Consider a real-life example like deleting a customer and their orders.

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