How many different product categories are there?
select distinct product_category_code from products
What unit of measurement is used for seeds category?
select unit_of_measure from ref_product_categories where product_category_code = 'Seeds'
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"