Entity Relationship Diagram
- 1. ENTITY RELATIONSHIP DIAGRAM (ERD) Chapter 4 ICT 2073 Prepared By: Siti Hajar Ismail
- 2. OBJECTIVES Define terms related to entity relationship modeling, including entity, entity instance, attribute, relationship and cardinality, and primary key. Describe the entity modeling process. Discuss how to draw an entity relationship diagram. Describe how to recognize entities, attributes, relationships, and cardinalities.
- 3. DATABASE MODEL 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
- 4. ENTITY RELATIONSHIP DIAGRAM (ERD) ER model allows us to sketch database designs ERD is a graphical tool for modeling data. ERD is widely used in database design ERD is a graphical representation of the logical structure of a database ERD is a model that identifies the concepts or entities that exist in a system and the relationships between those entities
- 5. PURPOSES OF ERD better understanding of the information to be contained in the database documentation tool. to communicate the logical structure of the database to users
- 6. COMPONENTS OF AN ERD An ERD typically consists of four different graphical components: 1. Entity 2. Relationship 3. Cardinality 4. Attribute
- 7. CLASSIFICATION OF RELATIONSHIP Optional Relationship An Employee may or may not be assigned to a Department A Patient may or may not be assigned to a Bed Mandatory Relationship Every Course must be taught by at least one Teacher Every mother have at least a Child
- 8. For a binary relationship set the mapping cardinality must be one of the following types: One to one A Manager Head one Department and vice versa One to many ( or many to one) An Employee Works in one Department or One Department has many Employees Many to many A Teacher Teaches many Students and A student is taught by many Teachers
- 12. CARDINALITY CONSTRAINTS EXAMPLE In our model, we wish to indicate that each school may enroll many students, or may not enroll any students at all. We also wish to indicate that each student attends exactly one school. The following diagram indicates this optionality and cardinality:
- 13. CARDINALITY CONSTRAINTS EXAMPLE (CONTD.) SCHOOL STUDENT Each school enrolls at least zero and at most many students Each student attends at least one and at most one school
- 14. GENERAL STEPS TO CREATE AN ERD Identify the entity Identify the entity's attributes Identify the Primary Keys Identify the relation between entities Identify the Cardinality constraint Draw the ERD Check the ERD
- 15. STEPS IN BUILDING AN ERD
- 16. A SIMPLE EXAMPLE A company has several departments. Each department has a supervisor and at least one employee. 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.
- 17. 1. IDENTIFY ENTITIES A company has several departments. Each department has a supervisor and at least one employee. 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. A true entity should have more than one instance
- 18. 2. FIND RELATIONSHIPS Aim is to identify the associations, the connections between pairs of entities. A simple approach to do this is using a relationship matrix (table) that has rows and columns for each of the identified entities.
- 19. FIND RELATIONSHIPS (CONTD.) Go through each cell and decide whether or not there is an association. For example, the first cell on the second row is used to indicate if there is a relationship between the entity "Employee" and the entity "Department".
- 20. 3. IDENTIFIED RELATIONSHIPS Names placed in the cells are meant to capture/describe the relationships. So you can use them like this 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
- 21. 4. DRAW ROUGH ERD Draw a diagram and: Place all the entities in rectangles Use diamonds and lines to represent the relationships between entities. General Examples
- 22. DRAWING ROUGH ERD (CONTD.)
- 23. DRAWING ROUGH ERD (CONTD.)
- 24. DRAWING ROUGH ERD (CONTD.)
- 25. 5. 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.
- 26. FILL IN CARDINALITY (CONTD.) The cardinality of a relationship can only have the following values One and only one One or more Zero or more Zero or one
- 28. CARDINALITY EXAMPLES 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
- 29. 6. ERD WITH CARDINALITY
- 30. EXAMPLES
- 31. ERD FOR COURSE ENROLLMENT
- 32. ERD FOR COURSE REGISTRATION
- 33. ROUGH ERD PLUS PRIMARY KEYS
- 34. 7. IDENTIFY ATTRIBUTES Identify and name all the attributes essential to the system we are studying without trying to match them to particular entities. The only attributes indicated are the names of the departments, projects, supervisors and employees, as well as the supervisor and employee NUMBER and a unique project number.
- 35. 8. MAP ATTRIBUTES For each attribute we need to match it with exactly one entity. Often it seems like an attribute should go with more than one entity (e.g. Name).
- 37. DRAW FULLY ATTRIBUTED ERD
- 38. 38 WEAK ENTITY SET Some entity sets in real world naturally depend on some other entity set They can be uniquely identified only if combined with another entity set Example: section1, section2, … become unique only if you put them into a context, e.g. csce4350
- 39. 39 WEAK ENTITY SET NOTATIONS Double rectangles for weak entity set Double diamond for weak entity relationship Dashed underscore for discriminator
- 40. LEARNING OUTCOME Define terms related to entity relationship modeling, including entity, entity instance, attribute, relationship and cardinality, and primary key. Explain the entity modeling process. Draw an entity relationship diagram.