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The
following normal forms are discussed in this section:
- First normal form: A table is in the first normal form if it
contains no repeating columns.
- Second normal form: A table is in the second normal form if
it is in the first normal form and contains only columns that are
dependent on the whole (primary) key.
- Third normal form: A table is in the third normal form if it
is in the second normal form and contains only columns that are
nontransitively dependent on the primary key.
When you follow these rules, the tables of the model are in the
third normal form, according to E. F. Codd,
the inventor of relational databases. When tables are not in the
third normal form, either redundant data exists in the model, or
problems exist when you attempt to update the tables.
If you cannot find a place for an attribute that observes these
rules, you have probably made one of the following errors:
- The attribute is not well defined.
- The attribute is derived, not direct.
- The attribute is really an entity or a relationship.
- Some entity or relationship is missing from the model.
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