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Relational Decomposition

  • When a relation in the relational model is not in appropriate normal form then the decomposition of a relation is required.
  • In a database, it breaks the table into multiple tables.
  • 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 join 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_ID EMP_NAME EMP_AGE EMP_CITY DEPT_ID DEPT_NAME
22 Denim 28 Mumbai 827 Sales
33 Alina 25 Delhi 438 Marketing
46 Stephan 30 Bangalore 869 Finance
52 Katherine 36 Mumbai 575 Production
60 Jack 40 Noida 678 Testing

The above relation is decomposed into two relations EMPLOYEE and DEPARTMENT

EMPLOYEE table:

EMP_ID EMP_NAME EMP_AGE EMP_CITY
22 Denim 28 Mumbai
33 Alina 25 Delhi
46 Stephan 30 Bangalore
52 Katherine 36 Mumbai
60 Jack 40 Noida

DEPARTMENT table

DEPT_ID EMP_ID DEPT_NAME
827 22 Sales
438 33 Marketing
869 46 Finance
575 52 Production
678 60 Testing

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

Employee ⋈ Department

EMP_ID EMP_NAME EMP_AGE EMP_CITY DEPT_ID DEPT_NAME
22 Denim 28 Mumbai 827 Sales
33 Alina 25 Delhi 438 Marketing
46 Stephan 30 Bangalore 869 Finance
52 Katherine 36 Mumbai 575 Production
60 Jack 40 Noida 678 Testing

Hence, the decomposition is Lossless join decomposition.

Dependency Preserving

  • It is an important constraint of the database.
  • In the dependency preservation, at least one decomposed table must satisfy every dependency.
  • If a relation R is decomposed into relation 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 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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