[housecarpenter]

As I understand it, the main advantage of having separate CUSTOMER and ORDER tables rather than just have a field on CUSTOMER which is a list of order IDs is that the latter structure makes it easy for someone querying the database to retrieve a list of order IDs for a given customer ID (just look up the value of the list-of-order-IDs field), but difficult to retrieve the customer ID for a given order ID (one would have to go through each customer and search through the list-of-order-IDs field for a match). The former structure, on the other hand, makes both tasks trivial to do with a JOIN.

[josephg]

What you're describing is a straightforward indexing problem. There's nothing stopping the database building an index which can look up customer IDs for a given order ID. You could even, if the database allows it, build a virtual ORDER table projection that you could query directly.