Fifth Normal Form (5NF)

Fifth Normal Form (5NF)

• It is also known as project-join normal form (PJ/NF).
• A database is said to be in 5NF, if and only if,
• The relation R should be already in 4NF.
• The relation R cannot be additionally non-loss decomposed (join dependency).
• What is Join dependency?
• Join dependency means joining two or more decomposed tables should not lose records nor create new records.
• Join Dependency can be associated to 5NF, wherein a relation is in 5NF, only if it is already in 4NF and it cannot be decomposed further.
• Example:
• If the join of R1 and R2 over S is equal to relation R then we can say that a join dependency exists, where R1 and R2 are the decomposition R1 (P, Q, S) and R2 (Q, T) of a given relation R (P, Q, S, T). 
• As a possibility, R1 and R2 are a lossless decomposition of R.
• Example:
• Consider a table which contains a record of Subject, Professor and Semester in three columns.

• The primary key is the combination of all three columns. 
• This table is in 4NF.
• It has no multivalued dependencies.
• It has no column is a determinant for the other two columns.
  
  
  
  
  
  
  

• A relation is in Fifth Normal Form (5NF), if it is in 4NF, and won’t have lossless decomposition into smaller tables.
• Hence, in this example, all the redundancies are eliminated, and the decomposition of the table is a lossless join decomposition. 
• Therefore, the relation is in 5NF as it does not violate the property of lossless join.