MORE INFORMATION
Description of normalization
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.
There are a few rules for
database normalization. Each rule is called a "normal form." If the first rule
is observed, the database is said to be in "first normal form." If the first
three rules are observed, the database is considered to be in "third normal
form." Although other levels of normalization are possible, third normal form
is considered the highest level necessary for most applications.
As
with many formal rules and specifications, real world scenarios do not always
allow for perfect compliance. In general, normalization requires additional
tables and some customers find this cumbersome. If you decide to violate one of
the first three rules of normalization, make sure that your application
anticipates any problems that could occur, such as redundant data and
inconsistent dependencies.
The following descriptions include
examples.
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.
It may be more feasible to apply third normal form only to data that
changes frequently. If some dependent fields remain, design your application to
require the user to verify all related fields when any one is changed.
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.
Normalizing an example table
These steps demonstrate the process of normalizing a fictitious
student table.
- Unnormalized table:
|
| 1022 | Jones | 412 | 101-07 | 143-01 | 159-02 |
| 4123 | Smith | 216 | 201-01 | 211-02 | 214-01 |
- First Normal Form: No Repeating Groups
Tables
should have only two dimensions. Since one student has several classes, these
classes should be listed in a separate table. Fields Class1, Class2, and Class3
in the above records are indications of design trouble.
Spreadsheets
often use the third dimension, but tables should not. Another way to look at
this problem is with a one-to-many relationship, do not put the one side and
the many side in the same table. Instead, create another table in first normal
form by eliminating the repeating group (Class#), as shown below:
|
| 1022 | Jones | 412 | 101-07 |
| 1022 | Jones | 412 | 143-01 |
| 1022 | Jones | 412 | 159-02 |
| 4123 | Smith | 216 | 201-01 |
| 4123 | Smith | 216 | 211-02 |
| 4123 | Smith | 216 | 214-01 |
- Second Normal Form: Eliminate Redundant Data
Note
the multiple Class# values for each Student# value in the above table. Class#
is not functionally dependent on Student# (primary key), so this relationship
is not in second normal form.
The following two tables demonstrate
second normal form:
Students
Registration
|
| 1022 | 101-07 |
| 1022 | 143-01 |
| 1022 | 159-02 |
| 4123 | 201-01 |
| 4123 | 211-02 |
| 4123 | 214-01 |
- Third Normal Form: Eliminate Data Not Dependent On
Key
In the last example, Adv-Room (the advisor's office number) is
functionally dependent on the Advisor attribute. The solution is to move that
attribute from the Students table to the Faculty table, as shown below:Students
Faculty