CREATE TABLE IF NOT EXISTS product_attributes (
    id INT AUTO_INCREMENT PRIMARY KEY,
    product_id INT NOT NULL,
    attribute_key_id INT NOT NULL,
    value TEXT NOT NULL,
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
    FOREIGN KEY (product_id) REFERENCES products(id) ON DELETE CASCADE,
    FOREIGN KEY (attribute_key_id) REFERENCES attribute_keys(id) ON DELETE CASCADE,
    UNIQUE KEY `product_attribute` (product_id, attribute_key_id)
);