Java Reference
In-Depth Information
Figure 2-1:
Foreign keys link the Client and Contacts Tables to the primary key of the
Address_Info table.
The tables conform to the requirements of the first normal form discussed in
Chapter
1
, for the following reasons:
All records in any one table have the same number of fields.
All fields contain only a single data item.
There are no repeated fields.
Finally, each row in the table is identified using a unique column or set of columns.
This unique identifier is the primary key.
The tables conform to the requirements of the second and third normal forms, which
are as follows:
The tables are in first normal form.
The tables cannot contain fields that are not related to the primary key.
The tables contain no fields that do not contain information related to the whole of the key.
Since the second normal form applies to tables that have a multipart key, and none of
these tables do, conformity is by default. However, multipart keys are not common.
Boyce-Codd normal form is a more rigorous version of the third normal form designed
to deal with tables containing:
Multiple candidate keys
Composite candidate keys
Candidate keys that ove rlap
As it turns out, the Client Table, with its Firm_ID and Tax_ID columns, has multiple
candidate keys. Assuming that the legacy Firm_ID column is unique, and knowing
that tax id codes should be unique, the Boyce-Codd normal form applies to this table.
