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
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.
ü It creates a link between the two.
ü The foreign key of the sub type is also its primary key.
Entities:
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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