To reduce
Table 9 to the third normal form, the Total After Tax column was eliminated because it contained data that can be derived. However, when data access requirements are considered, you discover that this data is constantly used. Although you can construct the Total After Tax value, your customer service representatives need this information immediately, and you do not want to have to calculate it every time it is needed. If it is kept in the database, it is always available on request. In this instance, the performance outweighs other considerations, so you denormalize the data by including the derived field in the table.
