DBMS Aggregation

DBMS Tutorial What is DBMS Types of DBMS Components of DBMS Architecture of DBMS Three Schema Architecture DBMS vs File System DBMS Languages DBMS ER Model Components of ER Diagram DBMS Generalization DBMS Specialization DBMS Aggregation Relational Model Codd's Rule in DBMS Relational Model in DBMS Integrity Constraints DBMS Keys DBMS Relational Algebra Convert ER Model into Tables DBMS Joins

In the previous two articles, we have discussed DBMS Generalization and Specialization. Here, we will learn about the third concept of Enhanced ER model, whose name is Aggregation.

What is Aggregation?

Aggregation is a concept which is used when the ER model cannot represent the relationship between an entity and the relationship. 

In the aggregation process, the relationship between the two entities is treated as a single entity. 

Example

Let's suppose there are two entities in the Hospital database, whose names are Doctor and Patient. 

These two entities are related through a relationship set Checks. 

A doctor who checks the patients must require the instruments or tools. So there will be another relationship set Instruments.  

We need to connect the relationship set requires to an entity set Instruments and relationship set Checks. But, we can connect only entity set to a relationship set.

One relationship set cannot be related to another relationship set; for this, we need the aggregation concept. So, aggregation is needed when you express a relationship set with another relationship set. 

In the following ER diagram, the Checks (relationship set) will treat like a higher-level entity and can be associated with a relationship set requires.