database-normalization

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.

1. Unnormalized table:
   
   

   Student#	Advisor	Adv-Room	Class1	Class2	Class3
   1022	Jones	412	101-07	143-01	159-02
   4123	Smith	216	201-01	211-02	214-01

2. 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:
   
   

   Student#	Advisor	Adv-Room	Class#
   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

3. 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:
   
   

   Student#	Advisor	Adv-Room
   1022	Jones	412
   4123	Smith	216

   
   
   Registration:
   
   

   Student#	Class#
   1022	101-07
   1022	143-01
   1022	159-02
   4123	201-01
   4123	211-02
   4123	214-01

4. 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:
   
   

   Student#	Advisor
   1022	Jones
   4123	Smith

   
   
   Faculty:
   
   

   Name	Room	Dept
   Jones	412	42
   Smith	216	42