DBMS Notes

Fifth Normal Form (5NF)

The Fifth normal form (5NF) is generally not implemented in real life database design. But we must learn the concept about it.

A relation will be in 5NF if

1. It is in 4NF

2. It does not hold any join dependency.

3. 5NF is also known as Project join normal form (PJ/NF).

Join Dependency

If a table holds the join dependency then

  • Decomposition of that table into multiple tables and re-joining of tables will always be same table as before decomposition. It is also called lossless decomposition.

If a table does not hold the join dependency then

  • Result of Decomposition or rejoining after decomposition leads toward either loss of data or new entries are created in the table. It is called lossy decomposition.

Concept of 5th Normal Form

  • We can understand the 5th NF by understanding either join dependency or breaking down the tables into parts and rejoin.
  • As Join dependency is a little bit confusing topic so let understand the breaking down the tables into parts.

Suppose a table SPC with composite primary key {Supplier, product, customer}

In the above table, supplier supplies products and customer can use these products but note that supplier does not directly supply to any customer.

In simple word, Supplier (“Ali”) produce (“ABC”) and Customer (“Nauman”) can use it. But Ali and Nauman are not directly connected.


After Decomposition of table (SPC) in to three parts as given below

Note that, In table (SC) Supplier and Customer are directly connected. So values are changed after decomposition of table (SPC) into parts.

So above table is in 5NF.

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