Third Normal Form
A database is in Third Normal Form (3NF) if it is in 2NF and every nonkey column is independent of every other nonkey column. In other words, the fields of a table other than the keys should be mutually independent.
If you followed the first two normalization steps properly, you will not necessarily need to make any changes at this stage. However, if you made a number of changes along the way (as can happen), this could be a last check. For example, say I wanted to record a contact name and email address with each invoice (Figure 3.8). The problem is that this information relates not to an invoice but to the client and, therefore, the database would fail the 3NF test. The correct structure would be to add these fields to the Clients table (Figure 3.9).
Figure 3.8 ltering the requirements of the database can muddle the design, as it is now no longer normalized with the Contact Name and Email Address additions.