I'm building an app that has a customers table, a locations table, a products table, and a product_locations table. They make a diamond shape. The locations table and the products table each have a customer_id column that links back to the customers table. Then the product_locations table table has just two columns: a location_id column and a product_id column, each linking back to the appropriate table. I want to write a constraint or a trigger or something else that makes sure that before a (location_id, product_id) tuple is inserted into the product_locations table, the system verifies that the product links to the same customer as the location. How do I do this? Thanks in advance. Matt
