6.29 Join Dependency And Projection Join Normal Form 5NF | fifth normal form | dbms tutorials

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Relational Database Design Functional Dependencies And Normalization


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Projection Join Normal Form (5NF) and Join Dependency
Join Dependency : Let R be a given relation upto 4NF and it decompose (Projected or
divided) into {R1, R2, R3, ..., Rn}. The relation R satisfy the join dependency * {R1, R2, R3, ...
Rn} if and only if joining of R1 to Rn = R.
226 Introduction to Database Management System
Consider the Relation XYZ with attributes X# (Customer_ID), Y# (Account_ID) and Z#
(Branch_ID) as shown in Figure 6.20.
First, decompose XYZ into three relations XY, YZ and ZX.
satisfy the Join dependency.
When joined, these three relations give original form of XYZ. So relation XYZ satisfy the
join dependency.
Functional Dependency and Normalisation 227
Definition Projection Join Normal Form (5NF) : Let R is a relation and D is a set of
all dependencies (Multivalued dependency, Functional dependency, Join dependency etc.) The
relation R is in 5NF w.r.t. D if every Join Dependency is trivial.
Or
A table is in fifth normal form (5NF) or ProjectJoin Normal Form (PJNF) if it is in 4NF
and it cannot have a lossless decomposition into any number of smaller tables.
Properties of 5NF: The following are the properties of 5NF
1. Anomalies can occur in relations in 4NF if the primary key has three or more fields.
2. 5NF is based on the concept of join dependence if a relation cannot be decomposed
any further then it is in 5NF.
3. Pairwise cyclical dependency means that:
l You always need to know two values (pairwise).
l For any one you must know the other two (cyclical).
5NF is the ultimate Normal form. A relation in 5 NF is guaranteed to be free of anomalies.
Consider the example in Figure 6.20, where relations XY, YZ and ZX are in 5NF.
Example. Consider the following Buying table. This table is used to track buyers, what
they buy, and from whom they buy? Take the following sample data:
Buyer Table
Buyer Vendor Item
Sam ITC Bath Soaps
Monika ITC Bath Soaps
Sam HCL Computer
Sam HCL Laptop
Monika HCL Computer
The problem with the above table is that if HCL starts to sell computers then how
many records must you create to record this fact? The problem is there as there are pairwise cyclical dependencies in the primary key. That is, in order to determine the item you
must know the buyer and vendor, and to determine the vendor you must know the buyer
and the item, and finally to know the buyer you must know the vendor and the item.
The solution is to break this one table into three tables; BuyerVendor table, BuyerItem
table and VendorItem table. All these three tables are in the 5NF.

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