Store derived data

A nice example of a useful derived data element is the balance of a bank account. You could derive it from all previous credit and debet statements, since the start balance 0. However, it would be very unpractical.

In the logical design, during UNF, Unnormalised Form you kept derived data seperate. You can use them now as a checklist for candicate derived data.

An example of derived data are totals and statistics, for example the number of readers in an area.

Attention Be careful with storing derived data. When someting changes in the source data, the derived data must change accordingly.

If the source data is very dynamic, it's better to just derive every time.