Conceptual Modeling Csc2507 Iv The Extended Entity -Books Pdf

Conceptual Modeling CSC2507 IV The Extended Entity
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Conceptual Modeling CSC2507,The Constructs of,the EER Model. 2004 John Mylopoulos The Extended Entity Relationship Model 3. Conceptual Modeling CSC2507, These represent classes of objects facts things people that have. properties in common and an autonomous existence, City Department Employee Purchase and Sale are examples of. entities for a commercial organization, An instance of an entity represents an object in the class represented by. the entity, Stockholm Helsinki are examples of instances of the entity City.
and the employees Peterson and Johanson are examples of. instances of the Employee entity, The EER model is very different from the relational model in a number of. ways for example in EER it is not possible to represent an object. without knowing its properties but in the relational model you need to. know its key attributes, 2004 John Mylopoulos The Extended Entity Relationship Model 4. Conceptual Modeling CSC2507,Examples of Entities, 2004 John Mylopoulos The Extended Entity Relationship Model 5. Conceptual Modeling CSC2507,Relationships, They represent logical links between two or more entities. Residence is an example of a relationship that can exist between the. entities City and Employee Exam is an example of a relationship. that can exist between the entities Student and Course. An instance of a relationship is an n tuple made up of instances of. entities one for each of the entities involved, The pair Johanssen Stockholm or the pair Peterson Oslo.
are examples of instances of the relationship Residence. 2004 John Mylopoulos The Extended Entity Relationship Model 6. Conceptual Modeling CSC2507,Examples of Relationships. 2004 John Mylopoulos The Extended Entity Relationship Model 7. Conceptual Modeling CSC2507,Example of Instances for Exam. 2004 John Mylopoulos The Extended Entity Relationship Model 8. Conceptual Modeling CSC2507,What Does An EER Diagram Really Mean. Course Meets Room, Course and Room are entities Their instances describe particular courses. e g CSC340S and particular rooms e g WB116, Meets is a relationship Its instances describe particular meetings Each.
meeting has exactly one associated course and room. Meets instances,Course instances,instances Room,Room instances. 2004 John Mylopoulos The Extended Entity Relationship Model 9. Conceptual Modeling CSC2507,Recursive Relationships. Recursive relationships are,also possible that is,relationships between an entity. and itself,Note in the second example,that the relationship is not. symmetric In this case it is,necessary to indicate the two.
roles that the entity involved,plays in the relationship. 2004 John Mylopoulos The Extended Entity Relationship Model 10. Conceptual Modeling CSC2507,Ternary Relationships, 2004 John Mylopoulos The Extended Entity Relationship Model 11. Conceptual Modeling CSC2507,AND XOR for Relationships. Contains Orders,either order a,Order XOR part or,Requests request a. Service service but,For any given,Shipment order,whenever there.
Order AND is at least one,invoice there is,Generates also at least one. Invoice shipment,and vice versa, 2004 John Mylopoulos The Extended Entity Relationship Model 12. Conceptual Modeling CSC2507,Shipment Part,FilledBy Contains. Generates Requests,Invoice Service,FilledBy Contains. Contains Part,Shipment FilledBy Order Part,Invoice Generates.
Generates Service, 2004 John Mylopoulos The Extended Entity Relationship Model 13. Conceptual Modeling CSC2507,Attributes, These describe the elementary properties of entities or relationships. For example Surname Salary and Age are possible attributes of. the Employee entity while Date and Mark are possible attributes for. the relationship Exam between Student and Course, An attribute associates with each instance of an entity or relationship. a value belonging to a set known as the domain of the attribute. The domain contains the admissible values for the attribute. 2004 John Mylopoulos The Extended Entity Relationship Model 14. Conceptual Modeling CSC2507,Attribute Examples, 2004 John Mylopoulos The Extended Entity Relationship Model 15. Conceptual Modeling CSC2507,Composite Attributes, It is sometimes convenient to group attributes of the same entity or.
relationship that have closely connected meanings or uses Such. groupings are called composite attributes, 2004 John Mylopoulos The Extended Entity Relationship Model 16. Conceptual Modeling CSC2507,Schema with Attributes. 2004 John Mylopoulos The Extended Entity Relationship Model 17. Conceptual Modeling CSC2507,Cardinalities, These are specified for each entity participating in a relationship and. describe the maximum and minimum number of relationship. occurrences in which an entity occurrence can participate. Cardinalities state how many times can an entity instance participate in. instances of a given relationship,An employee can participate in 1 to 5 assignments. A task can participate in 0 to 50 assignments, 2004 John Mylopoulos The Extended Entity Relationship Model 18.
Conceptual Modeling CSC2507,Cardinalities cont d, In principle a cardinality is any pair of non negative integers n m such. that n m or a pair of the form n N where N means any number. If minimum cardinality is 0 we say that entity participation in a. relationship is optional If minimum cardinality is 1 we say that entity. participation in a relationship is mandatory, If maximum cardinality is 1 each instance of the entity is associated at. most with a single instance of the relationship if maximum cardinality is. N then each instance of the entity is associated with an arbitrary. number of instances of the relationship, 2004 John Mylopoulos The Extended Entity Relationship Model 19. Conceptual Modeling CSC2507,Cardinality Examples, 2004 John Mylopoulos The Extended Entity Relationship Model 20. Conceptual Modeling CSC2507,Cardinality Example,meets three.
times a week,Course 3 3 0 40,Course Meets Room,A room can. A day can have up to 40,have an meetings per,unlimited 0 N week. 2004 John Mylopoulos The Extended Entity Relationship Model 21. Conceptual Modeling CSC2507,Instantiating ER Diagrams. An EER diagram specifies what states are possible in the world. that is being modeled,Course 2 2 0 40,Course Meets Room. 2004 John Mylopoulos The Extended Entity Relationship Model 22. Conceptual Modeling CSC2507,Illegal Instantiations.
Course 2 2 0 40,Course Meets Room, 2004 John Mylopoulos The Extended Entity Relationship Model 23. Conceptual Modeling CSC2507,Cardinalities of Attributes. They are specified for the attributes of entities or relationships and. describe the minimum and maximum number of values of the. attribute associated with instances of an entity or a relationship. In most cases the cardinality of an attribute is equal to 1 1 and is. omitted single valued attributes, The value of a certain attribute however may also be null or there. may exist several values of a certain attribute for an entity instance. multi valued attributes, 2004 John Mylopoulos The Extended Entity Relationship Model 24. Conceptual Modeling CSC2507,Cardinalities cont d, Multi valued attributes should be used with great caution because.
they represent situations that can be modelled in many cases with. additional entities linked by one to many or many to many. relationships to the entity to which they refer,Surname 0 N. Person Owns,Car Registration, 2004 John Mylopoulos The Extended Entity Relationship Model 25. Conceptual Modeling CSC2507,Identifiers, Identifiers or keys consist of one or more attributes which identify. uniquely instances of an entity, In many cases an identifier is formed by one or more attributes of. the entity itself in this case we talk about an internal identifier. Sometimes however the attributes of an entity are not sufficient to. identify its instances unambiguously and other entities are involved. in the identification Identifiers of this type are called external. identifiers, An identifier for a relationship consists of identifiers for all the entities.
it relates For example the identifier for the relationship Person. Owns Car is a combination of the Person and Car identifiers. 2004 John Mylopoulos The Extended Entity Relationship Model 26. Conceptual Modeling CSC2507,Examples of Identifiers. internal single attribute,internal multi attribute. external multi attribute, 2004 John Mylopoulos The Extended Entity Relationship Model 27. Conceptual Modeling CSC2507,General Observations on Identifiers. An identifier can involve one or more attributes provided that each of. them has 1 1 cardinality, An external identifier can involve one or more entities provided that.
each of them is member of a relationship to which the entity to. identify participates with cardinality equal to 1 1. An external identifier can involve an entity that is in its turn identified. externally as long as cycles are not generated, Each entity must have one internal or external identifier but can. have more than one Actually if there is more than one identifier. then the attributes and entities involved in an identification can be. optional minimum cardinality equal to 0, 2004 John Mylopoulos The Extended Entity Relationship Model 28. Conceptual Modeling CSC2507,Schema with Identifiers. 2004 John Mylopoulos The Extended Entity Relationship Model 29. Conceptual Modeling CSC2507,Modeling an Application with Identifiers. Identifiers constitute a powerful mechanism for modeling an application. Assume we want a database storing information about lecture meetings. Suppose first that we use the identifier coursename day. coursename day hour hour for the, Meeting entity This says that there can only be one meeting at any one.
time for a given course name day hour in other words we can t have two. sections of the same course meeting at the same day hour. Suppose now we use only coursename as identifier for Meeting. Meeting This says, that there can only be one meeting per given course name unreasonable. If we use courseinstructor room,courseinstructor room as identifier for Meeting. Meeting we are, stating that there can only be one meeting for a given instructor room. combination so an instructor must have all her meetings in different rooms. Finally if the attribute courseinstructor by itself forms an identifier for. Meeting then the diagram we have built is stating that each instructor. participates in at most one meeting again this is unreasonable. 2004 John Mylopoulos The Extended Entity Relationship Model 30.

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