In today’s post, we will learn how to make a Relational model from ER Diagram. ER diagram is just a conceptual picture of a database (Definition of ER Diagram HERE). It is one of the important tasks of the programmer to convert that diagram into tables while coding a database.
Let’s say we have to the ER Diagram of a company. The diagram has 3 strong entity sets, Employee, Department and Project with one-week entity Dependent.
How to make Relational model from ER Diagram
- Convert all string entity sets in to relations/tables.
- Exclude mutlivalues attribute from mapping into tables. Create a seperate relation for this . Add primary key of the entity set in new relation as a foreign key. The foreign key attribute and multivalued attribute will become composite key.
- Only consituent simple attributes will be mapped in case of composite attribute. For example Employee table has Name Composite attribute convert it into first_name, middle_name and Last_name.
- While mapping week entity set such as dependent, include primary key of the owner entity set into week entity set.
Mapping 1:1 Relationship Types
- Foreign Key Approach: Let R and S be two entities, find the entity with total participation (say S in the case). Add primary key of R into S as Foreign key
- Merged Relationship Approach: If both entity sets are having total participation then they can be merged into a single relation.
- Cross Reference Approach: Create a third relation having primary keys of both the entity sets.
Mapping 1:N Relationship Types
- Foreign Key Approach: Let R and S be two entities, find the entity with total participation (say S in the case). Add primary key of R into S as Foreign key. Usually from 1 Side to the N side.
Mapping M:N Relationship Types
- Create a third relation containing the primary keys of both the entity sets and descriptive attribute.
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