Foreign key ON UPDATE behavior in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to specify the action when the referenced key is updated.

SQL

CREATE TABLE orders (
  order_id INT PRIMARY KEY,
  customer_id INT,
  FOREIGN KEY (customer_id) REFERENCES customers(id) ON UPDATE [1]
);

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ACASCADE

BDELETE

CRESTRICT

DIGNORE

Attempts:

3 left

💡 Hint

Common Mistakes

Using DELETE instead of CASCADE causes errors because DELETE is not valid for ON UPDATE.

Using IGNORE is not a valid SQL action for ON UPDATE.

The ON UPDATE CASCADE option means that when the referenced key in the parent table changes, the foreign key in the child table updates automatically to match.

2fill in blank

medium

Complete the code to prevent updates to the referenced key if child rows exist.

SQL

CREATE TABLE enrollments (
  enrollment_id INT PRIMARY KEY,
  student_id INT,
  FOREIGN KEY (student_id) REFERENCES students(id) ON UPDATE [1]
);

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ACASCADE

BRESTRICT

CSET NULL

DNO ACTION

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing RESTRICT with CASCADE, which allows updates to propagate.

Using SET NULL here would set child keys to NULL, not restrict updates.

The ON UPDATE RESTRICT option prevents updating the referenced key if there are matching rows in the child table. It stops the update to keep data consistent.

3fill in blank

hard

Fix the error in the foreign key definition to correctly handle updates by setting child keys to NULL.

SQL

CREATE TABLE payments (
  payment_id INT PRIMARY KEY,
  order_id INT,
  FOREIGN KEY (order_id) REFERENCES orders(id) ON UPDATE [1]
);

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ASET NULL

BNO DELETE

CRESTRICT

DCASCADE

Attempts:

3 left

💡 Hint

Common Mistakes

Using NO DELETE is invalid syntax and causes errors.

CASCADE updates child keys to new values instead of setting NULL.

The ON UPDATE SET NULL option sets the foreign key in the child table to NULL when the referenced key in the parent table is updated. This requires the foreign key column to allow NULL values.

4fill in blank

hard

Fill both blanks to create a foreign key that updates child keys on parent update and deletes child rows on parent delete.

SQL

CREATE TABLE comments (
  comment_id INT PRIMARY KEY,
  post_id INT,
  FOREIGN KEY (post_id) REFERENCES posts(id) ON UPDATE [1] ON DELETE [2]
);

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ACASCADE

BSET NULL

CRESTRICT

DNO ACTION

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing CASCADE with RESTRICT can cause unexpected errors.

Using SET NULL on delete requires child keys to allow NULL.

Using ON UPDATE CASCADE updates child keys when the parent key changes. Using ON DELETE CASCADE deletes child rows when the parent row is deleted. This keeps data consistent automatically.

5fill in blank

hard

Fill all three blanks to define a foreign key that restricts updates, sets child keys to NULL on delete, and uses NO ACTION on update.

SQL

CREATE TABLE subscriptions (
  subscription_id INT PRIMARY KEY,
  user_id INT,
  FOREIGN KEY (user_id) REFERENCES users(id) ON UPDATE [1] ON DELETE [2] ON UPDATE [3]
);

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ARESTRICT

BSET NULL

CNO ACTION

DCASCADE

Attempts:

3 left

💡 Hint

Common Mistakes

Using CASCADE in all places can cause unwanted deletions or updates.

Confusing NO ACTION with RESTRICT; they behave differently in timing.

The foreign key uses ON UPDATE RESTRICT to block updates if child rows exist, ON DELETE SET NULL to set child keys to NULL when parent rows are deleted, and ON UPDATE NO ACTION to defer update checks until the end of the transaction.