What is the measurement used for chervil?
SELECT t2.unit_of_measure FROM products AS t1 JOIN ref_product_categories AS t2 ON t1.product_category_code   =   t2.product_category_code WHERE t1.product_name   =   "chervil"
measurement used for a product refers to unit_of_measure; relational join is used to connect products table and reference table for product categories;