1. 1 Data Modelling Which data to include in the database

2. 2 Aim of Data Modelling Form a model of the enterprise so that the data are accurate and useful. Yet another form of abstraction. First and most important step in database design. Requires a good understanding of enterprise being modeled. Different methods of data modeling, but the most widely used is the Entity-Relationship (E-R) model.

3. 3 Entity-Relationship Model Primarily used to specify an external view of the database. Recall that the conceptual schema can be created out of the various external views. Model the world in terms of entities relationships between entities attributes of entities and relationships.

4. 4 Elements in the E-R Model I Entity: A distinguishable thing, person or event Entity set A set of entities of the same type Relationship Representation of the fact that certain entities are related to each other in a specific way. Relationship set Set of relationships of a given type.

5. 5 Elements in the E-R Model II Attributes: Properties of entities or relationships Attribute values are drawn from a given domain or value set Attributes may have a set of values (multi-valued or set-valued attributes). Key: An attribute or set of attributes that uniquely identifies each entity in an entity set.

6. 6 E-R Diagrams E-R models can be visualized in E-R diagrams. There are different conventions for drawing E-R diagrams. The following is widely used: Rectangles for entity sets Ellipses for attributes Diamonds for relationship sets Lines to link Each component is labelled with its corresponding name.

7. 7 Example of an E-R Diagram I UWI offers many courses. Each course has a unique course number and a name. Students can enroll in many courses and a course can be taken by many students. A student has a name and a unique student number. Student obtain grades on courses. A course is taught by a single instructor.

8. 8 Example of an E-R Diagram II Course Instructor Student Teaches Enrol c num c name grade i num i name s num s_name

9. 9 Constraints I In addition to the entity and relationship sets and their attributes, during data modelling, one also identifies constraints. Examples: A course is taught by a single instructor. No student can enroll for more than 4 courses.

10. 10 Constraints II Usually, constraints are not captured in the E-R diagram and need to be written down. Two exceptions: Mapping cardinalities Concerns the number of entities involved in a relationship Existence dependencies Concerns the fact that sometimes the existence of an entity depends on the existence of another entity.

11. 11 One-to-One Relationships Each entity in E1 is related to at most one entity in E2 and vice versa. Example: A department has only one head and no person can be head of more than one department. Department Employee Head Of

12. 12 One-to-Many Relationships An entity in E2 is related to at most one entity in E1, but an entity in E1 can be related to many entities in E2. Example: A student has one professor as their supervisor but a professor may supervise many students. Student Professor Super vises

13. 13 Many-to-many Relationships An entity can be related to many entities in the other set, and vice versa. Example: A student can be enrolled in many courses, a course can have many students enrolled in it. Student Course Enroll grade

14. 14 Different Conventions There are different conventions for mapping cardinalities. For example, instead of a 1 on the link, one may use an arrow pointing into the entity. Department Employee Head Of

15. 15 Existence Dependencies An entity x is existence dependent on an entity y if the existence of x is dependent on the existence of y. Y is called the dominant entity; x the subordinate entity. Example: In a personnel database, a dependent (e.g., child) is existence dependent on the employee. If the employee is deleted, so is the dependent.

16. 16 Representation of Subordinate Entities Subordinate entities are usually represented in a double box Employee Dependent

17. 17 Some Complications to the E-R Diagram An entity set may stand in some relationship to itself. The links are then labelled with the role that the entity plays. Person Child-of ParentChild

18. 18 Multiple Relationships Between Entity Sets Course Student Enroll Tutor for

19. 19 Ternary Relationships A customer may has an account at a particular branch. Branch Customer Account CAB

20. 20 Generalizations Inheritance hierarchies Employee Academic Staff Support Staff IS A

21. 21 Specialization Sometimes, you want to make a lower level entity from a higher level entity. However, not every high-level entity must also be a lower-level entity. Student Grad Student IS A

22. 22 Aggregation Sometimes, you want to treat a relationship as an entity in its own right, e.g., to allow modelling of relationships between relationships. Example: A customer is assigned a loan manager for every loan that they get. Note that the relationship from the login is not with the customer alone, or with the loan.

23. 23 Aggregation in an E-R Model Customer Loan borrows Employee Loan-Manager

24. 24 Problems in Data Modelling Should a real-world concept be modelled as an entity set or a relationship set? Should a real-world concept be modelled as an attribute or an entity set? Is the use of a generalization or aggregation appropriate?