Best way to maintain a customer's account balance

Question

Is it better to have a field in the database that stores the customers account balance or use views and queries to generate the information.

Answer 1

"It depends". Most often, you want to avoid derived data. However, there are cases where having the derived total is justified.

Case in point:

I worked on a credit database application, where the balance was comprised of many different things, and different business rules over time. For example, the "balance" was actually a sum of different balances from different buckets, such as Principal, Fees, etc.

As transactions were posted, they were allocated to different buckets according to business rules. Fees went to the fees bucket. Purchases, credits, and debits went to the principal bucket. These allocations and rules were subject to change over time.

Imagine querying 100,000 customer balances on the fly in the face of changing business rules over time. This is a solid case where having the derived value actually make sense. We maintained a set of algorithms to "rewind" the account and reconstruct the balance chronologically for audit and verification purposes, but it was nothing you would want to do for large sets.