I want to link invoice table which contain list of product and service. The product data and the serice data share most common fields, eg. price, tax , but there also a few difference fields, eg. service_team_id for service data. I want to put product and service in different tables and merge it to create many-to-many relationship between invoice table and product&service. Is it possible to merge those 2 tables? If not, I have to put product and service in the same table and add a field as “type” (droplist: product and service) to distinguish them, which will use more stroage and unused fields for many records.