What are the scenarios for using triggers in database management systems?

Triggers in DBMS are special procedures that automatically execute upon certain events (insert, update, delete). Main scenarios of use:

• Automatic data integrity checks and maintenance. For example, automatically deleting related records in another table when a record is deleted.
• Logging changes. Recording the history of data modifications for audit purposes.
• Calculating and updating derived values. For example, updating a sum or status field when related data changes.
• Access restriction and validation. Checking conditions before data modification.

An example of a trigger in PostgreSQL that logs changes in the users table:

CREATE TRIGGER log_user_update
AFTER UPDATE ON users
FOR EACH ROW
EXECUTE FUNCTION log_update();

CREATE FUNCTION log_update() RETURNS trigger AS $$
BEGIN
  INSERT INTO user_changes(user_id, changed_at) VALUES (NEW.id, now());
  RETURN NEW;
END;
$$ LANGUAGE plpgsql;