ENTITY AND ATTRIBUTE
An entity is an object or component of data. An entity is represented as rectangle in an ER diagram.
For example: Let we have two entities Student and College and these two entities have many to one relationship as many students study in a single college.
An attribute describes the property of an entity. An attribute is represented as Oval in an ER diagram. There are four types of attributes:
1. Key attribute
2. Composite attribute
3. Multivalued attribute
4. Derived attribute
1. Key attribute:
A key attribute can uniquely identify an entity from an entity set. For example, student roll number can uniquely identify a student from a set of students. Key attribute is represented by oval same as other attributes however the text of key attribute is underlined.
2. Composite attribute:
An attribute that is a combination of other attributes is known as composite attribute. For example, In student entity, the student address is a composite attribute as an address is composed of other attributes such as pin code, state, country.
3. Multivalued attribute:
An attribute that can hold multiple values is known as multivalued attribute. It is represented with double ovals in an ER Diagram. For example – A person can have more than one phone numbers so the phone number attribute is multivalued.
4. Derived attribute:
A derived attribute is one whose value is dynamic and derived from another attribute. It is represented by dashed oval in an ER Diagram. For example – Person age is a derived attribute as it changes over time and can be derived from another attribute (Date of birth).
More topics from DBMS to read:
EasyExamNotes.com covered following topics in these notes.
- Introduction to Database
- Introduction to DBMS
- Advantages and disadvantages of DBMS
- DML, DDL and DCL
- Introduction to data models
- Entities and Attributes
- Relationship among entities
- Relational Database
- Twelve rules of CODD
- Integrity Constraints
- Functional dependency
- Transaction processing concepts
- OODBMS vs RDBMS
- SQL join
- SQL functions: SUM(), AVG(), MAX(), MIN(), COUNT().
- Block, Extent, Segment
- Oracle Background processes
- Oracle cursor
- Introduction to Concurrency Control
A list of Video lectures
- Korth, Silbertz,Sudarshan, “Fundamental of Database System”, McGraw Hill
- Atul Kahate , “ Introduction to Database Management System”, Pearson Educations