Explain the steps of an algorithm for ER to relational mapping
1 Answer
  • ER Model when conceptualized into diagrams gives a good overview of entity-relationship, which is easier to understand.
  • ER diagrams can be mapped to Relational schema using step by step procedure.
  • Though all the ER constraints cannot be imported into Relational model but an approximate schema can be generated.

ER Diagrams mainly comprised of:

  • Entity and its attributes
  • Relationship which is association among entities

Mapping Entity

An entity is a real world object with some attributes.

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  • Create table for each entity
  • Entity's attributes should become fields of tables with their respective data types.
  • Declare primary key

Mapping relationship

A relationship is association among entities.

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  • Create table for a relationship
  • Add the primary keys of all participating Entities as fields of table with their respective data types.
  • If relationship has any attribute, add each attribute as field of table.
  • Declare a primary key composing all the primary keys of participating entities.
  • Declare all foreign key constraints.

Mapping Weak Entity Sets

A weak entity sets is one which does not have any primary key associated with it.

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  • Create table for weak entity set
  • Add all its attributes to table as field
  • Add the primary key of identifying entity set
  • Declare all foreign key constraints

Mapping hierarchical entities

ER specialization or generalization comes in the form of hierarchical entity sets.

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  • Create tables for all higher level entities
  • Create tables for lower level entities
  • Add primary keys of higher level entities in the table of lower level entities
  • In lower level tables, add all other attributes of lower entities.
  • Declare primary key of higher level table the primary key for lower level table
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