Question

1. a. Explain why the functional dependency with the primary key as the determinant and all non-key attributes as dependents is present in any relation.

b. List reasons why we may have a relational model with data redundancy.

Answer 1

1a.

Functional Dependency basically determines the relation of one attribute to another attribute. It is typically between the Primary Key (PK) and the other non-key attributes within a table. For any relation R, attribute B is functionally dependent on attribute A (which is mostly the Primary Key)

Left Side (A) is called the determinant and the right side (B) is the dependent.

For Instance

Consider a relation R defined over a set of attributes (A1, A2,…..An)

and let X and Y be Í (A1, A2,……...An), then

X → Y

Y is functionally dependent on X if and only, whenever two tuples in R agree on their X value, they also agree on their Y value.

i.e if each X value in (A1, A2,…..An) has associated with it one Y value in (A1, A2,……..An)

If t1.X = t2.X then t1.Y = t2.Y

A determinant in a table is an attribute that can be used to determine the values assigned to other attributes in the same row. By this statement, any primary key is a determinant, but there may be determinants that are not the primary key.

1.b.

Data redundancy is the issue in organising the data which causes unnecessary duplication of data, thus utilising more space and if you want to make a change then you'll have to update many fields.

But data redundancy comes with a few advantages too in which we may have data redundancy are as follows:

• alternative data backup
• faster data access - beneficial in customer-service based organisation whose customers expect promptness
• better security
• improved data reliability

Reasons that cause data redundancy is that when the same piece of data is stored in two or more separate places. It occurs when the data is not normalised and it causes anomalies like insert, delete and update.

Data redundancy increases the cost as well.