Entity Sets
What is an Entity Setsin DBMS:
An entity is an object of real world so set of entities of the same type.
Example : A person in an enterprise is an entity and he has one attribute which uniquely identify him that is P_ID personal id which is 18zX13
Similarly a loan entity with Loan_no as its attribute and let L_No is L578. So here we can say ' set of all persons who are customers at a given Bank can be defined as ' entity set customer' similarly an entity set of loan might represent set of all loans.
Entity set do not need to be disjoint
In this an employee in the bank maybe also customer in that Bank.
Attribute: Entities are described by a set of attributes.
Example an employee or customer set -> have attributes Like Customer_Id, Customer_Name, Customer_Street, and Customer-City.
Similarity a loan entity set attributes like
- loan_number
- loan_amount
Customer_id attribute in customer entity set is our primary key which means through value of it we can uniquely access particular row because
Customer name: It can be same name of other person
Customer City: Many customer may have same city
Domain or Value Set:
For each attribute, there is a set of permitted values which is called "Domain or Value Set". The domain of attribute "Customer_Name" might be set of all text strings of certain length. Similarly domain of attribute "Loan-Number" might be set of Strings of form 'L-n' Where n is a positive integer.
Entity can be described by set of (attribute, data values)
Example:(customer_ID, 677-89-9011) for customer entity set(customer name, Deepak)...... This is done for all entities for their all attributes.
