IS 4420 Database Fundamentals Chapter 3: Modeling Data in the Organization Leon Chen

1. IS 4420Database Fundamentals Chapter 3:Modeling Data in the Organization Leon Chen

2. Systems Development Life Cycle Database Development Process Enterprise modeling Project Identification and Selection Conceptual data modeling Project Initiation and Planning Analysis Logical Design Logical database design Physical database design and definition Physical Design Implementation Database implementation Maintenance Database maintenance

3. Overview • What is data modeling • Importance of data modeling • Business rules • Entity-relationship model: entities, relationships, and attributes

4. Conceptual Data Modeling • Business Rules • Data names • Data definitions Conceptual Data Model (Entity-Relationship Diagram)

5. Data Modeling is the Most Important • Input for follow-up processes • Data rather than processes are the most complex • Data tend to be more stable than business processes

6. Business Rules • Statements that define or constrain some aspect of the business. For example: • Friday is business casual dress day • Students can only register a course if prerequisite is successfully completed • Automated through DBMS software

7. Entity-Relationship Model • A logical representation of the data for an organization or for a business area • 3 constructs: • Entity - person, place, object, event, concept (often corresponds to a row in a table) • Attribute - property or characteristic of an entity (often corresponds to a field in a table) • Relationship – link between entities (corresponds to primary key-foreign key equivalencies in related tables) • Type vs. instance • Entity type: Instructor • Entity instance: Leon Chen • Entity-Relationship Diagram • A graphical representation of entity-relationship model • Also called E-R diagram or just ERD STUDENT Name Registers

8. Sample E-R Diagram (Figure 3-1) Attribute Supplier_ID Supplier_Name Supplier_Address

9. A special entity that is also a relationship Relationship symbols Attribute symbols Entity symbols Relationship degrees specify number of entity types involved Relationship cardinalities specify how many of each entity type is allowed

10. Entity • Person: EMPLOYEE, STUDENT • Place: WAREHOUSE, COUNTRY • Object: BUILDING, MACHINE • Event: SALE, REGISTRATION • Concept: ACCOUNT, COURSE CERTIFICATE EMPLOYEE DEPENDENT Weak entity Strong entity Associative entity

11. What Should an Entity Be? • SHOULD BE: • An object that will have many instances in the database • An object that will be composed of multiple attributes • An object that we are trying to model • SHOULD NOT BE: • A user of the database system • An output of the database system (e.g. a report)

12. Figure 3-4 Inappropriate entities System user System output Appropriate entities

13. Attribute • Attribute - property or characteristic of an entity type • Classifications of attributes: • Required vs. Optional Attributes • Simple vs. Composite Attribute • Single-Valued vs. Multivalued Attribute • Stored vs. Derived Attributes • Identifier Attributes - keys

14. Required vs. Optional Attributes • Example: entity – ONLINE_ACCOUNT • Required attributes (Not NULL) • Account_ID • Password • Owner_Name • Optional attributes • Phone_Number • Password_Hint

15. Figure 3-7 – A composite attribute An attribute broken into component parts

16. Derived from date employed and current date Multivalued: an employee can have more than one skill Figure 3-8 – Entity with a multivalued attribute (Skill) and derived attribute (Years_Employed)

17. Figure 3-19 – An attribute that is both multivalued and composite This is an example of time-stamping. More examples?

18. Identifiers (Keys) • Identifier (Key) - An attribute (or combination of attributes) that uniquely identifies individual instances of an entity type • Candidate Key – an attribute that could be a key. Examples (for STUDENT, PERSON)? • Simple Key versus Composite Key

19. The key is underlined Figure 3-9a Simple key attribute

20. Figure 3-9b Composite key attribute Flight_Number Date

21. Guidelines for Identifiers • Will not change in value • Will not be null • Substitute new, simple keys for long, composite keys • ?  Game_Number

22. Relationship • Relationship Type vs. Relationship Instance • Degree of a relationship • Cardinality of a relationship • Associative Entity – combination of relationship and entity

23. attribute of the relationship Figure 3-11a The relationship type is modeled as the diamond and lines between entity types

24. Relationship Instanceis between specific entity instances

25. One entity related to another of the same entity type Entities of two different types related to each other Entities of three different types related to each other Degree of a relationship is the number of entity types that participate in it

26. Cardinality of a Relationship • One-to-One • Each entity in the relationship will have exactly one related entity • One-to-Many • An entity on one side of the relationship can have many related entities, but an entity on the other side will have a maximum of one related entity • Many-to-Many • Entities on both sides of the relationship can have many related entities on the other side

29. Note: a relationship can have attributes of its own

30. Cardinality Constraints • Cardinality Constraints - the number of instances of one entity that can or must be associated with each instance of another entity • Minimum Cardinality. Examples? • If zero, then optional • If one or more, then mandatory • Maximum Cardinality. Examples? • The maximum number

31. Cardinalities Mandatory one Mandatory many Optional one Optional many 11 18

32. Figure 3-16a Basic relationship Mandatory minimum cardinalities – Figure 3-17a

33. Figure 3-17c Optional cardinalities with unary degree, one-to-one relationship

34. Maximum Cardinality Constraint max cardinality constraint

35. Entities can be related to one another in more than one way

36. Attributes or Relationship? Attributes can be transformed to relationships

37. Attributes or Relationship? Attributes should be transformed to relationships

38. Associative Entities • It’s an entity– it has attributes • AND it’s a relationship– it links entities together • When should a relationship with attributes be an associative entity? • All relationships for the associative entity should be many • The associative entity could have meaning independent of the other entities • The associative entity should have at least one or more attributes other than the identifier • The associative entity may participate in other relationships other than the entities of the associated relationship • Ternary relationships should be converted to associative entities

39. Figure 3-11b – An associative entity (CERTIFICATE)

40. Bill of materials structure

41. Figure 3-18 – Ternary relationship as an associative entity

42. Strong vs. Weak Entities • Strong entities • exist independently of other types of entities • has its own unique identifier • represented with single-line rectangle • Weak entity • dependent on a strong entity…cannot exist on its own • does not have a unique identifier • represented with double-line rectangle • Identifying relationship • links strong entities to weak entities • represented with double line diamond

43. Strong entity Identifying relationship Weak entity

44. Figure 3-22a E-R diagram for Pine Valley Furniture

45. Microsoft Visio Notation for Pine Valley Furniture Different modeling software tools may have different notation for the same constructs

46. A special entity that is also a relationship Relationship symbols Attribute symbols Entity symbols Relationship degrees specify number of entity types involved Relationship cardinalities specify how many of each entity type is allowed

47. Sample E-R Diagram (Figure 3-1) Attribute Supplier_ID Supplier_Name Supplier_Address