Data Redundancyis common problem which our big organization facing today. Now I will explain you what is Data Redundancy:-
Ø Redundancy means repetition of data.
Ø Redundancy increases the time involved in updating, adding, and deleting data.
Ø It also increases the utilization of disk space and hence, disk I/O increases.
Ø Redundancy can lead to the following problems:
v Inserting, modifying, and deleting data may cause inconsistencies
v Errors are more likely to occur when facts are repeated
v Unnecessary utilization of extra disk space
Normalization
Ø Normalization is a scientific method of breaking down complex Table structures into simple table structures by using certain rules.
Ø It allows you to reduce redundancy in a table and eliminate the problems of inconsistency and disk space usage.
Ø Normalization results in the formation of tables that satisfy certain specified rules and represent certain normal forms.
Ø The most important and widely used normal forms are:
v First Normal Form (1NF)
v Second Normal Form (2NF)
v Third Normal Form (3NF)
v Boyce-Codd Normal Form (BCNF)
First Normal Form (1 NF)
Ø A table is said to be in the 1NF when each cell of the table contains precisely one value.
Ø Functional Dependency:
v The normalization theory is based on the fundamental notion of functional dependency.
Second Normal Form (2 NF)
Ø A table is said to be in 2NF when it is in 1NF and every attribute in the row is Functionally Dependent upon the whole key, and not just part of the key.
Ø To ensure that a table is in 2NF, you should:
v Find and remove attributes that are functionally dependent on only a part of the key and not on the whole key and place them in a different table.
v Group the remaining attributes.
Third Normal Form (3 NF)
Ø A relation is said to be in 3NF when it is in 2NF and every non-key attribute is functionally dependent only on the primary key.
Ø To ensure that a table is in 3NF, you should:
v Find and remove non-key attributes that are functionally dependent on attributes that are not the primary key and place them in a different table.
v Group the remaining attributes.
Boyce-Codd Normal Form (BCNF)
Ø The original definition of 3NF was inadequate in some situations.
Ø It was not satisfactory for the tables:
v that had multiple candidate keys
v where the multiple candidate keys were composite
v where the multiple candidate keys overlapped
Ø Therefore, a new normal form-the BCNF was introduced:
v A relation is in the BCNF if and only if every determinant is a candidate key.
Ø In order to ensure that a table is in BCNF, you should:
v Find and remove the overlapping candidate keys.
v Place the part of the candidate key and the attribute it is functionally dependent on, in a different table.
v Group the remaining items into a table.
Denormalization
Ø The end product of normalization is a set of related tables that comprise the database.
Ø However, in the interests of speed of response to critical queries, which demand information from more than one table, it is sometimes wiser to introduce a degree of redundancy in tables.
Ø The intentional introduction of redundancy in a table to improve performance is called denormalization.
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