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08 April, 2022

Relational Decomposition

 Programing Coderfunda     April 08, 2022     DBMS     No comments   

 

Relational Decomposition

  • When a relation in the relational model is not relationship normal form then the decomposition of a relation is required.
  • In a database, it breaks the table into multiple tablinappropriatees.
  • If the relation has no proper decomposition, then it may lead to problems like loss of information.
  • Decomposition is used to eliminate some of the problems of bad design like anomalies, inconsistencies, and redundancy.

Types of Decomposition


DBMS Relational Decomposition

Lossless Decomposition

  • If the information is not lost from the relation that is decomposed, then the decomposition will be lossless.
  • The lossless decomposition guarantees that the joining of relations will result in the same relation as it was decomposed.
  • The relation is said to be lossless decomposition if natural joins of all the decomposition give the original relation.

Example:

EMPLOYEE_DEPARTMENT table:

EMP_IDEMP_NAMEEMP_AGEEMP_CITYDEPT_IDDEPT_NAME
22Denim28Mumbai827Sales
33Alina25Delhi438Marketing
46Stephan30Bangalore869Finance
52Katherine36Mumbai575Production
60Jack40Noida678Testing

The above relation is decomposed into two relations EMPLOYEE and DEPARTMENT

EMPLOYEE table:

EMP_IDEMP_NAMEEMP_AGEEMP_CITY
22Denim28Mumbai
33Alina25Delhi
46Stephan30Bangalore
52Katherine36Mumbai
60Jack40Noida

DEPARTMENT table

DEPT_IDEMP_IDDEPT_NAME
82722Sales
43833Marketing
86946Finance
57552Production
67860Testing

Now, when these two relations are joined on the common column "EMP_ID", then the resultant relation will look like this:

Employee ⋈ Department

EMP_IDEMP_NAMEEMP_AGEEMP_CITYDEPT_IDDEPT_NAME
22Denim28Mumbai827Sales
33Alina25Delhi438Marketing
46Stephan30Bangalore869Finance
52Katherine36Mumbai575Production
60Jack40Noida678Testing

Hence, the decomposition is Lossless join decomposition.

Dependency Preserving

  • It is an important constraint of the database.
  • Independence preservation, at least one decomposed table must satisfy every dependency.
  • If a relation R is decomposed into relations R1 and R2, then the dependencies of R either must be a part of R1 or R2 or must be derivable from the combination of functional dependencies of R1 and R2.
  • For example, suppose there is a relation R (A, B, C, D) with a functional dependency set (A->BC). The relational R is decomposed into R1(ABC) and R2(AD) which is dependency preserving because FD A->BC is a part of relation R1(ABC).
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