Explain how to reduce a relationship set of an E-R diagram into relational schema.
There are several processes and algorithms available to convert ER Diagrams into Relational Schema. Some of them are automated and some of them are manual. We may focus here on the mapping diagram contents to relational basics. ER diagrams mainly comprise of −
An entity is a real-world object with some attributes.
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 an association among entities.
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 set is one which does not have any primary key associated with it.
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.
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-level entities.
Declare primary key of higher-level table and the primary key for lower-level table.
Declare foreign key constraints.
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