14 June 2011

Normalization with Examples

Normalization with Examples

Normalization is the process of organizing data in a database.
This includes creating tables and establishing relationships between those tables according to rules designed both to protect
the data and to make the database more flexible by eliminating redundancy and inconsistent dependency.

Redundant data wastes disk space and creates maintenance problems.
If data that exists in more than one place must be changed, the data must be changed in exactly the same way in all locations. A customer address change is much easier to implement if that data is stored only in the Customers table and nowhere else in the database.

What is an "inconsistent dependency"? While it is intuitive for a user to look in the Customers table for the
address of a particular customer, it may not make sense to look there for the salary of the employee who calls on that customer.
The employee's salary is related to, or dependent on, the employee and thus should be moved to the Employees table.
Inconsistent dependencies can make data difficult to access because the path to find the data may be missing or broken.


First Normal Form

Eliminate repeating groups in individual tables.
Create a separate table for each set of related data.
Identify each set of related data with a primary key.
Do not use multiple fields in a single table to store similar data. For example, to track an inventory item that may come from two
possible sources, an inventory record may contain fields for Vendor Code 1 and Vendor Code 2.

What happens when you add a third vendor? Adding a field is not the answer; it requires program and table modifications and
does not smoothly accommodate a dynamic number of vendors. Instead, place all vendor information in a separate table called Vendors,
then link inventory to vendors with an item number key, or vendors to inventory with a vendor code key.

Second Normal Form

Create separate tables for sets of values that apply to multiple records.
Relate these tables with a foreign key.
Records should not depend on anything other than a table's primary key (a compound key, if necessary).
For example, consider a customer's address in an accounting system. The address is needed by the Customers table, but also by the Orders,
Shipping, Invoices, Accounts Receivable, and Collections tables. Instead of storing the customer's address as a separate entry in each
of these tables, store it in one place, either in the Customers table or in a separate Addresses table.

Third Normal Form

Eliminate fields that do not depend on the key.
Values in a record that are not part of that record's key do not belong in the table. In general, any time the contents of a group
of fields may apply to more than a single record in the table, consider placing those fields in a separate table.

For example, in an Employee Recruitment table, a candidate's university name and address may be included. But you need a complete list
of universities for group mailings. If university information is stored in the Candidates table, there is no way to list universities
with no current candidates. Create a separate Universities table and link it to the Candidates table with a university code key.

EXCEPTION: Adhering to the third normal form, while theoretically desirable, is not always practical.
If you have a Customers table and you want to eliminate all possible interfield dependencies,
you must create separate tables for cities, ZIP codes, sales representatives, customer classes, and any other factor that may be
duplicated in multiple records. In theory, normalization is worth pursing. However, many small tables may degrade performance or
exceed open file and memory capacities.


Other Normalization Forms

Fourth normal form, also called Boyce Codd Normal Form (BCNF), and fifth normal form do exist, but are rarely considered in practical
design. Disregarding these rules may result in less than perfect database design, but should not affect functionality.

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