1. Database Modeling Using the Entity-Relationship Model(Continued)

2. Usage of Weak Entities • Weak Entity can be used when converting a n-ary relationship to a number of binary relationships. • The basic algorithm can be found in the previous lecture note. • The additional ENTITY CLASS is actually a weak entity.

3. SName ProjName Quantity SUPPLY PartNo An Example of Ternary Relationship SUPPLIER PROJECT PART

4. SName ProjName Quantity SUPPLY SP SS SPJ PartNo Converted E-R Diagram N 1 N 1 SUPPLIER PROJECT N 1 PART

5. SName ProjName Quantity SUPPLY PartNo What if defining a cardinality constraint 1 M SUPPLIER PROJECT N PART

6. SName ProjName Quantity SUPPLY SP SPJ PartNo Converted E-R Diagram N 1 1 1 SUPPLIER PROJECT SS N 1 PART

7. Design Principles • Faithfulness • Avoiding redundancy • Keep it simple • Picking the right kind of element

8. Faithfulness • Include everything needs to be described. • Avoid meaningless descriptions. • Enforce as many constraints as possible. • Cardinality Ratios. • Participation Constraints • Existence Dependency.

9. Cardinality Ratios • The cardinality ratios for binary relationships • 1:1, 1:N, N:1, and M:N • Attributes of 1:1 relationship types can be migrated to either of the participating entity types. • Attributes of 1:N or N:1 relationship types can be migrated only to the entity type at N-side of the relationship. • Attributes of M:N relationship types must be specified as relationship attributes.

10. Starting date Starting date Starting date Examples Manages DEPARTMENT EMPLOYEE 1 1 Manages DEPARTMENT EMPLOYEE 1 1 Manages DEPARTMENT EMPLOYEE 1 1

11. Hours Hours Works_on Works_on Examples N 1 EMPLOYEE PROJECT 1 N EMPLOYEE PROJECT Note: IF we assumed that each employee can only work on one project, and there may be many employees working on the same project, THEN: The attribute Hours can be migrated to EMPLOYEE, but not PROJECT.

12. Hours Examples Works_on EMPLOYEE PROJECT N M Note: IF we assume that each employee may work on multiple projects at the same time, and there may be many employees working on the same projects, THEN: The attribute Hours can not be migrated to either PROJECT or EMPLOYEE.

13. Participation Constraints • It specifies whether the existence of an entity depends on its being related to another entity via relationship type. • Two types • total participation (Using double line connecting the participating entity type) • partial participation • Total participation is also called Existence Dependency.

14. Works_for Examples N 1 DEPARTMENT EMPLOYEE Note: IF we assume that every employee should be assigned to certain department. THEN: There is a total participation constraint defined on EMPLOYEE via the relationship Works_for.

15. Works_for Examples N 1 DEPARTMENT EMPLOYEE Note: IF we assume that each department should have at least one employee being assigned, THEN: There is a total participation constraint defined on DEPARTMENT via the relationship Works_for.

16. Examples 1 1 Manages DEPARTMENT EMPLOYEE Note: IF we assume that should have exactly one manager, but not every employee could be a manager of certain department, THEN: There is a total participation constraint defined on DEPARTMENT via the relationship type Manages. However, EMPLOYEE partially participates the relationship type Manages.

17. Alternative Notations for Constraints • A pair of integer (min, max) with each participation of entity type E in a relationship type R. ( maxmin 0, max 1) • min: at least min; max: at most max • min = 0 implies partial participation. • min 1 implies total participation.

18. Works_for Works_for Examples (1,1) (4,N) DEPARTMENT EMPLOYEE This example implies that each employee is assigned to one and only one department. Each department has at least four employees, and there is no upper limit of the number of employees in a department. N 1 DEPARTMENT EMPLOYEE

19. Examples (0,1) (1,1) DEPARTMENT Manages EMPLOYEE What does this example imply? 1 1 Manages DEPARTMENT EMPLOYEE What does this example imply?

20. Dependents_of DEPENDENT sex birthdate Examples (0,N) EMPLOYEE (1,1) SSN name relationship What does this example imply?

21. Avoiding Redundancy • Problem with redundancy • Wastes space. • Encourages inconsistency. • Principle: describe and store everything once only. • Intuition: something is redundant if it could be hidden from view, and you could still figure out what it is from the other data.

22. Examples name name address BEERS ManfBy MANFS name manf manfadd BEERS Question: Which one is better?

23. KISS: Keep It Simple, Student • Avoid intermediate concepts if unnecessary. name name BEERS ManfBy MANFS