Redundancy and Inconsistent dependency

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

What is an “inconsistent dependency"? Although it is intuitive for a user to look into the Customers table for the address of an exacting customer it should not make sense to look there for the salary of the employee who calls on that customer. The employee's salary is associated to, or dependent on, the employee and therefore should be moved to the Employees table. To Inconsistent dependencies can make data hard to access; the path to find the data should be missing or broken.

There are a few rules for database normalization. Every rule is known as a normal form. If the first rule is observed then the database is said to be in first normal form. If the first three rules are observed then the database is considered to be in third normal form. While other levels of normalization are probably third normal form is taken as the highest level must for most applications.

The three rules of Normalization are as follows:

1.No repeating groups of attributes (fields).

Note: Erased all the duplicating info (attributes). Create a new entity for those.

2.Every non-key attribute (field) is fully functionally dependent upon the key field.

Note: Erased those attributes which are dependent on other attributes.

3.No non-key fields are dependent upon the other non-key field(s).

Note: Erased those attributes that come out by itself (ie.,) automatically.

Let us described this with an instance

Consider an object known as Invoice that contains attributes which are needed to prepare an Invoice.

Invoice Object Attributes

Invoice_code, Inv_Date, Order_Date,order_qty, Order_code, Cust_code,cust_name, Address, item_code, invoice_value, description, unitcost,ordervalue,