Third Normal Form

The third normal form denotes that there must not be any dependency among non-key attributes. Resolving the above we get,

Invoice_Master        Invoice_Item        Item_Master

Invoice_code           Invoice_No           Item_code

Inv_date                       item_code            description

Order_code                   description            rate

Invoice_Value                 ord_qty

                                  Ord_value

Order_Cust            Customer_Master

Order_code                cust_code    

Cust_code                  cust_name  

Order_date                  address    

Third normal form is achieved when transitive dependencies are erased from a record design. A number of the non-key attributes are dependent not only on the primary key but also on a non-key attribute. That’s referred to as a transitive dependency.

Conversion to 3rd normal form removes transitive dependence through splitting the association into two relations.

Motivation for concern. When there is a transitive dependence the deletion of A will cause deletion of B and C as well.

•A, B and C are three data items in a record
•If functionally dependency of C is on B and
•B is functionally dependent on A
•Then C is functionally dependent on A
•Thus, a transitive dependency exists.

It is also in third normal form because there are no transitive dependencies.