- 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
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
A relationship is 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 sets 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 entities.
- Declare primary key of higher level table the primary key for lower level table