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ALTER TABLE categories ADD FOREIGN KEY (parent_id) REFERENCES categories(id) ON DELETE SET NULL;
ALTER TABLE products ADD FOREIGN KEY (category_id) REFERENCES categories(id) ON DELETE SET NULL;
ALTER TABLE product_expirations ADD FOREIGN KEY (product_id) REFERENCES products(id) ON DELETE CASCADE;
ALTER TABLE reviews ADD FOREIGN KEY (product_id) REFERENCES products(id) ON DELETE CASCADE;
ALTER TABLE reviews ADD FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE;
ALTER TABLE product_images ADD FOREIGN KEY (product_id) REFERENCES products(id) ON DELETE CASCADE;