1 Chapter 11 Entity-Relationship Modeling Transparencies Last Updated: 25 April 2011 By M. Arief

2 Database Design u Approaches include: –Bottom-up »Attributes  Relation, Entities –Top-down »Relation, Entities  Attributes –Inside-out »Attributes  Major entities  Other entities –Mixed

3 Chapter 11 - Objectives u How to use Entity–Relationship (ER) modeling in database design. u Basic concepts associated with ER model. u Diagrammatic technique for displaying ER model using Unified Modeling Language (UML).

4 ER Diagram of Branch View of DreamHome

5 Concepts of the ER Model u Entity types u Relationship types u Attributes

6 Entity Type u Entity type –Group of “objects” with same properties, identified by enterprise as having an independent existence. u Object - something that is or is capable of being seen, touched, or otherwise sensed, and about which users store data and associate behavior. u Entity occurrence –Uniquely identifiable object of an entity type.

7 Examples of Entity Types

8 Example of entity occurrence Student IDLast NameFirst Name 2144ArnoldBetty 3122TaylorJohn 3843SimmonsLisa 9844MacyBill 2837LeathHeather 2293WrenchTim Entity occurrence Entity: student

9 ER Diagram of Staff and Branch Entity Types UML: First letter of each word is in upper case Ex. PropertyForRent

10 Relationship Types u Relationship type –Set of meaningful associations among entity types. u Relationship occurrence –Uniquely identifiable association, which includes one occurrence from each participating entity type.

11 Semantic Net of Has Relationship Type

12 ER Diagram of Branch Has Staff Relationship UML: - First letter of each word is in upper case Ex. LeasedBy - Use arrow symbol

13 Relationship Types u Degree of a Relationship –Number of participating entities in relationship. u Relationship of degree: –two is binary; –three is ternary; –four is quaternary.

14 Binary Relationship called POwns

15 Ternary Relationship called Registers Other example: Project, Location, Employee, Assignment

16 Quaternary Relationship called Arranges

17 Relationship Types u Recursive Relationship –Relationship type where same entity type participates more than once in different roles  unary relationship. u Relationships may be given role names to indicate purpose that each participating entity type plays in a relationship.

18 Recursive Relationship called Supervises with Role Names

19 Other example of Recursive Relationship

20 Entities associated through two distinct Relationships with Role Names

21 Attributes u Attribute –Property of an entity or a relationship type. »UML: first letter of first word is in lower case, first letter of other words is in upper case u Attribute Domain –Set of allowable values for one or more attributes.

22 Attributes u Simple Attribute –Attribute composed of a single component with an independent existence. »Ex. position, salary u Composite Attribute –Attribute composed of multiple components, each with an independent existence. »Ex. address: street+city+postcode

23 Attributes u Single-valued Attribute –Attribute that holds a single value for each occurrence of an entity type. »Ex. branchNo u Multi-valued Attribute –Attribute that holds multiple values for each occurrence of an entity type. »Ex. UML: telNo [1..3]

24 Attributes u Derived Attribute –Attribute that represents a value that is derivable from value of a related attribute, or set of attributes, not necessarily in the same entity type. »Ex. /leasedDuration= rentStart – rentFinish UML: /totalSalary, /totalStaff

25 Keys u Candidate Key –Minimal set of attributes that uniquely identifies each occurrence of an entity type. u Primary Key –Candidate key selected to uniquely identify each occurrence of an entity type. »UML: branchNo {PK} u Composite Key –A candidate key that consists of two or more attributes. »Ex. Advert(propertyNo, newspaperName, dateAdvert, cost) Composite key: propertyNo+newspaperName,+dateAdvert

26 ER Diagram of Staff and Branch Entities and their Attributes

27 Entity Type u Strong Entity Type (parent) –Entity type that is not existence-dependent on some other entity type. OR –Each participating entity has its own independent primary key. u Weak Entity Type (child) –Entity type that is existence-dependent on some other entity type. OR –The parent entity’ key is also part of the primary key of the child entity.

28 Strong Entity Type called Client and Weak Entity Type called Preference

29 Strong and Weak Entity Weak Entity Strong Entity

30 Strong and Weak Entity

31 Relationship called Advertises with Attributes Indicate that that the relationship conceals unidentified entity type

32 Structural Constraints u Main type of constraint on relationships is called multiplicity. u Multiplicity - number (or range) of possible occurrences of an entity type that may relate to a single occurrence of an associated entity type through a particular relationship. u Represents policies (called business rules) established by user or company.

33 Structural Constraints u The most common degree for relationships is binary. u Binary relationships are generally referred to as being: –one-to-one (1:1) –one-to-many (1:*) –many-to-many (*:*)

34 Semantic Net of Staff Manages Branch Relationship Type

35 Multiplicity of Staff Manages Branch (1:1) Relationship Type

36 Semantic Net of Staff Oversees PropertyForRent Relationship Type

37 Multiplicity of Staff Oversees PropertyForRent (1:*) Relationship Type

38 Semantic Net of Newspaper Advertises PropertyForRent Relationship Type

39 Multiplicity of Newspaper Advertises PropertyForRent (*:*) Relationship

40 Structural Constraints u Multiplicity for Complex Relationships –Number (or range) of possible occurrences of an entity type in an n-ary relationship when other (n-1) values are fixed.

41 Semantic Net of Ternary Registers Relationship with Values for Staff and Branch Entities Fixed

42 CR56/B003 CR57/B003 CR62/B003 CR84/B003 CR91/B003 r1 r2 r3 r4 r5 SG37 SG5 SG14

43 Multiplicity of Ternary Registers Relationship

44 Summary of Multiplicity Constraints

45 Structural Constraints u Multiplicity is made up of two types of restrictions on relationships: cardinality and participation. u Cardinality –Describes maximum number of possible relationship occurrences for an entity participating in a given relationship type. u Participation –Determines whether all or only some entity occurrences participate in a relationship.

46 Multiplicity as Cardinality and Participation Constraints

47

48

49 Introduction to the UML Unified Modeling Language (UML) – a set of modeling conventions that is used to specify or describe a software system in terms of objects. – The UML does not prescribe a method for developing systems—only a notation that is now widely accepted as a standard for object modeling.