Chapter 8 Entity-Relationship Modeling Pearson Education © 2009

2 Chapter 12 - 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). u How to identify and resolve problems with ER models called connection traps. u How to build an ER model from a requirements specification. Pearson Education © 2009

3 ER diagram of Branch user views of DreamHome Pearson Education © 2009

4 Concepts of the ER Model u Entity types u Relationship types u Attributes Pearson Education © 2009

5 Entity Type u Entity type –Group of objects with same properties, identified by enterprise as having an independent existence. –Can be objects with physical existence or objects with a conceptual existence. Pearson Education © 2009

6 Entity Type u Entity occurrence –Uniquely identifiable object of an entity type. u Each entity type is shown as a rectangle u Labelled with the name of the entity - Noun Pearson Education © 2009

7 ER diagram of Staff and Branch entity types Pearson Education © 2009

8 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. u Each relationship type is shown as a line connecting the associated entity types and labelled with the name of relationship u Using verb / short phrase including verb Pearson Education © 2009

9 Semantic net of Has relationship type Pearson Education © 2009 u Semantic Net u Object-level model u Symbol. to represent entities and symbol to represent relationships u Represent Branch and Staff using values for the primary key attributes

10 ER diagram of Branch Has Staff relationship Pearson Education © 2009

11 Relationship Types u Degree of a Relationship –Number of participating entities in relationship. u The entities involved in a particular relationship type are referred to as participants u The number of participants in a relationship type is called the degree of that relationship u Relationship of degree : –two is binary –three is ternary –four is quaternary. Pearson Education © 2009

12 Binary relationship called POwns Pearson Education © 2009

13 Ternary relationship called Registers Pearson Education © 2009

14 Quaternary relationship called Arranges Pearson Education © 2009

15 Relationship Types u Recursive Relationship –Relationship type where same entity type participates more than once in different roles. –Sometimes called as unary u Relationships may be given role names to indicate purpose that each participating entity type plays in a relationship. Pearson Education © 2009

16 Recursive relationship called Supervises with role names Pearson Education © 2009

17 Entities associated through two distinct relationships with role names Pearson Education © 2009

18 Attributes u Attribute –Property of an entity or a relationship type. u Attribute Domain –Set of allowable values for one or more attributes. –Potential values that an attribute may hold –Attributes may share a domain –Domains can also be composed of domains Pearson Education © 2009

19 Attributes u Simple Attribute/Atomic –Attribute composed of a single component with an independent existence. –Cannot be further subdivided into smaller components –examples: position, salary, employeeID u Composite Attribute –Attribute composed of multiple components, each with an independent existence. –Address: »Street, City and Postcode Pearson Education © 2009

20 Attributes u Single-valued Attribute –Attribute that holds a single value for each occurrence of an entity type. u Multi-valued Attribute –Attribute that holds multiple values for each occurrence of an entity type. Pearson Education © 2009

21 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. Pearson Education © 2009

22 Keys u Candidate Key –Minimal set of attributes that uniquely identifies each occurrence of an entity type. –Must hold values that are unique for every occurrence of an entity type u Primary Key –Candidate key selected to uniquely identify each occurrence of an entity type. u Composite Key –A candidate key that consists of two or more attributes. Pearson Education © 2009

23 ER diagram of Staff and Branch entities and their attributes Pearson Education © 2009

24 Entity Type u Strong Entity Type –Entity type that is not existence-dependent on some other entity type. –Sometimes referred to as parent, owner or dominant u Weak Entity Type –Entity type that is existence-dependent on some other entity type. –Sometimes referred to as child, dependent, subordinate Pearson Education © 2009

25 Strong entity type called Client and weak entity type called Preference Pearson Education © 2009

26 Relationship called Advertises with attributes Pearson Education © 2009 u Attributes can also be assigned to relationships

27 Structural Constraints u Constraints that may be placed on entity types that participate in a relationship u Constraints should reflect the restrictions on the relationships as perceived in the real world u Examples: –Property for rent must have an owner and each branch must have staff Pearson Education © 2009

28 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. Pearson Education © 2009

29 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) – a member of staff manages a branch –one-to-many (1:*) – a member of staff oversees properties for rent –many-to-many (*:*) – newspapers advertise properties for rent Pearson Education © 2009

30 Semantic net of Staff Manages Branch relationship type Pearson Education © 2009

31 Multiplicity of Staff Manages Branch (1:1) relationship Pearson Education © 2009

32 Semantic net of Staff Oversees PropertyForRent relationship type Pearson Education © 2009

33 Multiplicity of Staff Oversees PropertyForRent (1:*) relationship type Pearson Education © 2009

34 Semantic net of Newspaper Advertises PropertyForRent relationship type Pearson Education © 2009

35 Multiplicity of Newspaper Advertises PropertyForRent (*:*) relationship Pearson Education © 2009

36 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. Pearson Education © 2009

37 Semantic net of ternary Registers relationship with values for Staff and Branch entities fixed Pearson Education © 2009

38 Multiplicity of ternary Registers relationship Pearson Education © 2009

39 Summary of multiplicity constraints Pearson Education © 2009

40 Structural Constraints u Multiplicity is made up of two types of restrictions on relationships: cardinality and participation. Pearson Education © 2009

41 Structural Constraints 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. Pearson Education © 2009

42 Multiplicity as cardinality and participation constraints Pearson Education © 2009

43 Problems with ER Models u Problems may arise when designing a conceptual data model called connection traps. u Often due to a misinterpretation of the meaning of certain relationships. u Two main types of connection traps are called fan traps and chasm traps. Pearson Education © 2009

44 Problems with ER Models u Fan Trap –Where a model represents a relationship between entity types, but pathway between certain entity occurrences is ambiguous. u Chasm Trap –Where a model suggests the existence of a relationship between entity types, but pathway does not exist between certain entity occurrences. Pearson Education © 2009

45 An Example of a Fan Trap Pearson Education © 2009 u Fan Trap: Where a model represents a relationship between entity types, but pathway between certain entity occurrences is ambiguous. u Fan trap may exist where two or more 1:* relationships fan out from the same entity u A single division operates one or more branches and has one or more staff u Which members of staff work at a particular branch?

46 Semantic Net of ER Model with Fan Trap u At which branch office does staff number SG37 work? Pearson Education © 2009

47 Restructuring ER model to remove Fan Trap Pearson Education © 2009

48 Semantic Net of Restructured ER Model with Fan Trap Removed u SG37 works at branch B003. Pearson Education © 2009

49 An Example of a Chasm Trap Pearson Education © 2009 u Chasm Trap: Where a model suggests the existence of a relationship between entity types, but pathway does not exist between certain entity occurrences. u Chasm trap may occur where there are one or more relationships with a minimum multiplicity of zero forming part of the pathway between related entities. u A single branch has one or more staff who oversee zero or more properties for rent.

50 An Example of a Chasm Trap Pearson Education © 2009 u Not all staff oversee property and not all properties are overseen by a member of staff u Which properties are available at each branch?

51 Semantic Net of ER Model with Chasm Trap u At which branch office is property PA14 available? Pearson Education © 2009

52 ER Model restructured to remove Chasm Trap Pearson Education © 2009

53 Semantic Net of Restructured ER Model with Chasm Trap Removed Pearson Education © 2009