Progress
Database Design
Guide
The Third Normal Form
A table is in the third normal form when it contains only independent columns, that is, columns not derived from other columns.
The third rule of normalization is that you must remove columns that can be derived from existing columns.
Table 2–8 shows an Order table with a Total After Tax column that is calculated from adding a 10% tax to the Order Amount column.
To reduce this table to third normal form, eliminate the Total After Tax column because it is a dependent column that changes when the Order Amount or tax change. For your report, you can create an algorithm to obtain the amount for Total After Tax. You need only keep the source value because you can always derive dependent values. Similarly, if you have an Employee table, you do not have to include an Age column if you already have a Date of Birth column, because you can always calculate the age from the date of birth.
A table that is in third normal form gives you these advantages:
Although a database normalized to the third normal form is desirable because it provides a high level of consistency, it may impact performance when you physically implement the database. When this occurs, consider denormalizing these tables. "Database Design Basics" discusses denormalization.
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