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Relational Database Management System (Part-3)

Conceptual Model

Ø Reflects entities and their relationships, based on the data processing needs of an organisation
Ø Is independent of individual applications, Database Management systems, hardware, and physical storage of data

ER to Tables

ER diagram can be represented by a collection of tables in the relational system.

Regular Entities
Ø “Building blocks” of the Database. Each regular Entity maps to a table.

Weak Entities
Ø a weak entity is an entity whose existence depends on some other entity.
Ø It cannot exist if the entity on which it depends does not exist.

Sub types and Super types
Ø Each entity type (sub types and super types) maps to a separate table.
Ø the primary key of the super type is the Foreign Key of the sub type:
  ü It creates a link between the two.
  ü The foreign key of the sub type is also its primary key.

Some attributes may acquire further attributes during database design and become entities.

Ø Sub entities:
ü      Replace optional attributes of an entity with sub entities. This is called specialization.
ü      Specialization is the result of taking a subset of a higher-level entity set to form a lower-level entity set.
ü      Generalization is the result of taking the union of two or more lower-level entity sets to produce a higher‑level entity set.

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Relational Database Management System (Part-3)


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