Boyce/Codd Normal Form
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It is just an extension to the third normal form. Third normal form ensures that non key attributes does not depend on any non key attribute but fully depend on the key[primary] attributes.
Boyce codd normal form ensures that non key attributes are fully dependent on the total set of the primary key. Non-key attributes should not depend [only] on a subset of the key[primary] attributes.
Fourth Normal Form:
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It tries to decompose an entity/table into multiple entities/tables when there are multiple independent[not dependent on each other] multivalued attributes in an entity/table.
For example: Consider a table with emp_number, skill name, objective of the employee. Employees may have many skills and also many objectives. Hence, skill_name, and objective are multivalued and also they do not depend on each other. This table will cause many redundant data.Better is to use two tables like:
Emp_skill
Emp_num, Skill Name
Emp_objective
Emp_num, Objective
From: http://sitestree.com/?p=4762
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Post Data:2012-01-14 02:58:00
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