MySQL - Calculating Values vs Storing in Fields

Question

I apologize if this has been asked before, but I'm pretty new to this and unable to find an answer that addresses the situation I'm faced with.

I'm trying to put together a database to run behind our company website. The database will store information on customer invoices and payments. I'm trying to figure out if I should create a field for the invoice balance, or if I should just have it calculate when the customer account is accessed? I don't want to create redundant data, and don't want to have the chance that somehow the field wouldn't be updated, and would thus be incorrect...but I also don't want to create to large of a burden on the server - especially if we pull up an overview of customer accounts - which would need to then calculate the balance of every account. Right now we are starting from scratch, so I want to set it up right!

We are anticipating having a couple hundred customer accounts by the end of the year, but will most likely be up to a couple thousand by the end of next year. (Average number of invoices per customer would be roughly 2-3 per year.)

Answer 1

Invoice balance is essential data to store, however I think you meant account balance since you referred to that later.

Storing the account balance would be denormalizing it, and that's not how accounting databases are typically designed. Always calculate account balance from invoices minus payments. Denormalizing is almost always a bad idea, and if you need to optimize in the future, there are other places to cache data that are more efficient than the database.

In your use case, a query like that on a few thousand rows would be negligible anyway, so don't optimize before you have to.

Answer 2

There are probably other things to consider as well. For example, what if your invoice consist of ID's of products in another table... and the prices of those other products change? When you go to generate the invoice, you'll have the wrong total in there for what the guy actually paid 6 months ago. So if its a situation like that, you'll probably want to store the total on the invoice. And I wouldn't worry too much about doing a little math if you go the other route, it's not likely to be a huge bottleneck.

Answer 3

Yes, remember that items/goods could and will change their prices over time. You need to have the invoice balance as of the day of the purchase. Calculating the balance on the fly could lead to wrong balances later on.