# Minimization of ER Diagram

## Problem statement

ER diagram is pictorial representation of shows various tables and relations amongst them. ER diagram we can reduce the number of database.

## One to one cardinality

Let us consider below diagram with one to one cardinality −

Above ER diagram represents 3 entities −

• Employee entity has 2 attributes namely emp_name. emp_id is the primary key
• Company entity has 2 attributes namely cmp_name. cmp_id is the primary key
• Primary key of Work entity can be emp_id or cmp_id
• We cannot combine 3 tables into single one can either merge Work into Employee or Company. minimum 2 tables are required in one to one cardinality scenario.

## One to many cardinality

Let us consider below diagram with one to many cardinality −

In this ER diagram employee can work for one company but company can have many such employees. Above ER diagram represents 3 entities −

• Employee entity has 2 attributes namely − emp_id and emp_name. emp_id is the primary key
• Company entity has 2 attributes namely − emp_id and cmp_name. cmp_id is the primary key
• We cannot make cmp_id as primary key as multiple employee can work for same cmp_id. However, we can combine Employee and Work table. Hence minimum 2 tables are required in one to many cardinality scenario.

## Many to many cardinality

Let us consider below diagram with one to many cardinality −

In this ER diagram employee can work for multiple companies a company can have many such employees. Above ER diagram represents 3 entities −

• Employee entity has 2 attributes namely − emp_id and emp_name. emp_id is the primary key
• Company entity has 2 attributes namely − emp_id and cmp_name. cmp_id is the primary key
• Unlike previous example cannot merge Work table with either Employee or Company. If we try to merge it then it will create redundant data.Hence minimum 2 tables are required in many to many cardinality scenario

Updated on: 22-Oct-2019

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