Boyce - Codd - Every determinant in table must be candidate key - Database

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• Nirja Shah   -Posted on 03 Nov 15
  - A technique of organising data is known as database Normalisation.
  
  - It is a systematic approach of decomposing tables to eliminate data redundancy and undesirable characteristics like Insertion, Update and Deletion Anamolies.
  
  - There is a multi-step process followed that puts data into tabular form by removing duplicated data from the relation tables.
  
  - It is mainly used for two purpose:
  1. Eliminating the reduntant(useless) data.
  2. Ensuring data dependencies make sense i.e data is logically stored.
  
  1 NF
  - In this form, no two Rows of data must contain repeating group of information i.e each set of column must have a unique value, such that multiple columns cannot be used to fetch the same row.
  - Every table in the database should be organized into rows, and each row should have a primary key that distinguishes it as unique.
  - The Primary key is usually a single column, but sometimes a combination of more than one column is used to create a single primary key.
  
  2NF
  - In this form there must not be any partial dependency of any column on primary key.
  - It means that for a table that has concatenated primary key, each column in the table that is not part of the primary key must depend upon the entire concatenated key for its existence.
  - A table fails Second normal form when any column depends only on one part of the concatenated key.
  
  4NF
  - This normal form is used in database normalization.
  - It is the next level of normalization after Boyce–Codd normal form (BCNF).
  - Whereas the second, third, and Boyce–Codd normal forms are concerned with functional dependencies, 4NF is concerned with a more general type of dependency known as a multivalued dependency.
  - A Table is in 4NF if and only if, for every one of its non-trivial multivalued dependencies X \⇒ Y, X is a superkey—that is, X is either a candidate key or a superset thereof.
  
  BCNF
  - This normal form is a higher version of the third normal form.
  - It deals with certain type of anamoly that is not handled by 3NF.
  - A 3NF table which does not have multiple overlapping candidate keys is said to be in BCNF.

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