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Entity Relationship Diagram

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ERD

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Entity Relationship Diagram

  1. 1. Entity Relationship Diagram (ERD) Presented by, RAKHI MUKHERJI BSc. Part-3 (Computer Hons.) 12181SC102
  2. 2. OBJECTIVES  To understand What is an ERD & why it is used?  Define terms related to entity relationship modeling, including entity, relationship ,cardinality, attributes ,primary key.  Describe the steps involved in entity relationship diagram.  Describe how to recognize entities, attributes, relationships, and cardinalities.  Discuss how to draw an entity relationship diagram with the help of an example.
  3. 3. * A database can be modeled as: *a collection of entities, *relationship among entities. Database systems are often modeled using an Entity Relationship (ER) diagram as the "blueprint" from which the actual data is stored — the output of the design phase. The ERD is used to communicate the logical structure of the database to users
  4. 4. ERD is a model that identifies the entities that exist in a system and the relationships between those entities ERD is a graphical tool for modeling data. ER model allows us to sketch database designs
  5. 5. Components of an ERD An ERD consists of four components Entity Relationship Cardinality Attributes
  6. 6. BASIC CONCEPTS
  7. 7. WHAT IS ENTITY? • Person, place, object, event or concept about which data is to be maintained • Examples of entities: – Person: EMPLOYEE, STUDENT, PATIENT – Place: STORE, WAREHOUSE – Object: MACHINE, PRODUCT, CAR – Event: SALE,REGISTRATION, RENEWAL – Concept: ACCOUNT, COURSE
  8. 8. WHAT IS ENTITY? • Person
  9. 9. WHAT IS ENTITY? • Place
  10. 10. WHAT IS ENTITY? • Object
  11. 11. WHAT IS ENTITY? • Event
  12. 12.  Associations between instances of one or more entity types that is of interest Meaningful association among several entities.  Given a name that describes its function. Author Book Writes
  13. 13.  Unary Relationship: between two instances of one entity type. Student Cheats from
  14. 14. Binary Relationship : between the instances of two entity types. Teacher Teaches Subject
  15. 15.  Ternary Relationship : among the instances of three entity types
  16. 16. A A A A B B B B Each instance of A is related to a minimum of zero and a maximum of one instance of B Each instance of B is related to a minimum of one and a maximum of one instance of A Each instance of A is related to a minimum of one and a maximum of many instances of B Each instance of B is related to a minimum of zero and a maximum of many instances of A • Cardinality : minimum and maximum number of instances of Entity B that can (or must be) associated with each instance of entity A.
  17. 17. CARDINALITY EXAMPLE SCHOOL STUDENT Each school enrolls at least zero and at most many students Each student attends at least one and at most one school
  18. 18. ATTRIBUTES Descriptive properties possessed by each member of an entity set. Example of entity types and associated attributes: Student: Student_name Employee: Employee_age Event: Event_location
  19. 19. Student Name Gender Class Roll number Age Primary Key: An attribute that has been selected as the unique identifying characteristic for an entity type. Example: Student: Student_rollnumber Staff: Staff_Id
  20. 20. General Steps to create an ERD Identify the entity Identify the relation between entities Identify the Cardinality constraint Identify the entity's attributes Draw the ERD
  21. 21. A Simple Example A company has several departments. Each department has a supervisor and at least one employee. Every supervisor has only one department under him. Employees must be assigned to at least one, but possibly more departments. At least one employee is assigned to a project, but an employee may be on vacation and not assigned to any projects. The important data fields are the names of the departments, projects, supervisors and employees, as well as the supervisor and employee number and a unique project number.
  22. 22. • A company has several departments. Each department has a supervisor and at least one employee. Every supervisor has only one department under him Employees must be assigned to at least one, but possibly more departments. At least one employee is assigned to a project, but an employee may be on vacation and not assigned to any projects. The important data fields are the names of the departments, projects, supervisors and employees, as well as the supervisor and employee number and a unique project number. • A true entity should have more than one instance
  23. 23. A Department is assigned an employee A Department is run by a supervisor An employee belongs to a department An employee works on a project A supervisor runs a department A project uses an employee
  24. 24. Fill in Cardinality • Supervisor – Each department has one supervisor. • Department – Each supervisor has one department. – Each employee can belong to one or more departments • Employee – Each department must have one or more employees – Each project must have one or more employees • Project – Each employee can have 0 or more projects.
  25. 25. ERD with Cardinality
  26. 26. Employee Supervisor Project • Employee Name • Employee No. • Supervisor Name • Supervisor No. • Project Name • Project No. Department Department Name ATTRIBUTES
  27. 27. Rough ERD Plus Primary Keys
  28. 28. Fully Attributed ERD Department Is Assign Supervisor Employee ProjectWorks On Runs By Department Name Project Name Project Number Employee Number Supervisor Number Supervisor Name Employee Name

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