Question

I am a bit indecisive about a certain database table structure, here is the scenario:

I have an “invoices” table, and an invoice can be for a “product” or for a “service” (products are in a table, and services in a separate table).

What is the best way to link an invoice to either of those, but also allow adding more options? If it was an invoice for only a product, the table design would be:

id, product_id, amount

But what is the best way for this scenario? I was thinking something like:

id, source_id, invoice_type (product or service), amount

But I thought that it might not be a good idea.

Answer 1

If there aren’t any differences between product and service invoices, just use one table with an invoice_type field.

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After reading your entire question, what you listed at the end is right. What you should look at is examples of a subclass and superclass.

Basically, all your fields that are the same can be grouped into one table, then fields that only pertain to a specific type of service can be grouped into its own table. Instead of having 1 or 2 tables, you’ll end up with a max of 3 (a superclass and 2 subclasses where needed), and they will be more normalized.

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Given your example (Note: this is just an example):

• Superclass:Invoice

  id, invoice_type (product or service), amount, invoice_dt, customer_paid
  

• Subclass:Product

  invoice_id, msrp, cost, purchase_dt
  

• Subclass:Services

  invoice_id, duration, extraneous_cost, start_dt, end_dt