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The Entity Relationship Model

The Entity Relationship Model. The Entity Relationship Model. Outline. ER model Overview Entity types Attributes, keys Relationship types Weak entity types uses Integration DEFinition for Information Modeling (IDEF1X) notation in Erwin EER model Subclasses Specialization/Generalization

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The Entity Relationship Model

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  1. The Entity Relationship Model The Entity Relationship Model --The Entity Relationship Model(1)--

  2. Outline • ER model • Overview • Entity types • Attributes, keys • Relationship types • Weak entity types • uses Integration DEFinition for Information Modeling (IDEF1X) notation in Erwin • EER model • Subclasses • Specialization/Generalization • Schema Design • Single DB • View integration in IS --The Entity Relationship Model(1)--

  3. Data Modelling Process Mini-world requirements collection and analysis database requirements conceptual design conceptual schema DBMS- independent data model mapping logical schema DBMS- specific physical design internal schema --The Entity Relationship Model(1)--

  4. Entity-Relationship Model • This model is used in conceptual design. • Popular in CASE tools, e.g., SSADM, DataArchitect • A database can be modelled as • a collection of entity types, and • relationships among entity types. • Result is an ER Schema or an ER Diagram. --The Entity Relationship Model(1)--

  5. MiniWorld Definition: A miniworld is a part of the real world that we are interested in modelling. --The Entity Relationship Model(1)--

  6. Entity Definition: An entity is an object that exists and is distinguishable from other objects. • Object, exists, distinguishable • DVD store mini-world example • Physically exist The CUSTOMER Jane Doe. The Xtra-visionSTORE. The EMPLOYEE Juan Gonzales. --The Entity Relationship Model(1)--

  7. Entity, cont. • Abstract or organisational entities that do not exist physically • The Introduction to Databases COURSE. • The SCHOOL of Computing. • Events • The TUTORIAL on Friday, October 31, 2007. • The EXAMINATION on Tuesday December 15, 2007. --The Entity Relationship Model(1)--

  8. Attribute Definition: An attribute is a property of an entity. • Feature example: CUSTOMER Name Date of Birth (DOB) example:DVD Copy number Title • CUSTOMERentity examples Name=Jane, DOB=1/1/70 Name=Joe,DOB=5/6/75 --The Entity Relationship Model(1)--

  9. Entity Type Definition: An entity set is a set of entities of the same type. Definition: An entity type is a description of the attributes that a set of entities has in common. • Entity set: a set of actual entities • Entity type: the ideal description of a kind of entity • Example:A CUSTOMER entity type. A CUSTOMER has a name and a date of birth (DOB). Name=Joe, DOB=5/6/75 Name=Julius, DOB=9/7/55 CUSTOMER entities CUSTOMER entity sets Name=Jane, DOB=1/1/70 Name=Fred, DOB=8/2/64

  10. Entity Type, cont. • Focus on attributes of an ideal entity • An entity has a value for every attribute of the entity type • Use a null value to represent the following. • An unknown attribute value A Customer’s Address is unknown. • An inapplicable attribute value A Customer’s Phone number is inapplicable if that person does not own a phone. CUSTOMER entities unknown date of birth Name=Jane, DOB=@ Name=Fred, DOB=8/2/64

  11. Distinguishing Entities • How can entities be distinguished from each other? • Assumption: Each entity will have a unique combination of attribute values. • Example, assume the Customer entity type has Name and DOB attributes. There may be several Customers with a Name of ‘Joe’. Also, there may be several Customers with a DOB of 5/6/75. But there can be at most oneCustomer with the Name ‘Joe’ and a DOB of 5/6/75. • Is there a smaller such set of attributes? CUSTOMER entities Name=Joe, DOB=5/6/75 Name=Joe, DOB=10/7/49 Name=Fred, DOB=5/6/75

  12. Keys Definition: A super key of an entity type is a set of one or more attributes whose values uniquely determine each entity. • Assume eachCustomerhas a uniqueID. The following are super keys. {Name, DOB, ID}, {Name, ID}, {Name, DOB}, {DOB, ID}, {ID} Definition: A candidate key is a minimal super key. • {Name, DOB} is a candidate key ofCustomer. • {ID} is a candidate key ofCustomer. Definition: Generally, there are may be several candidate keys. One of the keys is selected to be the primary key. • {ID} is the primary key ofCustomer. --The Entity Relationship Model(1)--

  13. ER Diagram - Diamond Notation • An entity type is represented with a labelled rectangle. • An attribute is represented with a labelled oval, connected to an entity type with a line. Customer DOB Customer Name --The Entity Relationship Model(1)--

  14. Attributes - Diamond Notation • Single valued versus multi-valued • E.g., multiple telephone extensions • Simple attributes versus structured attributes • E.g., an address is composed of a number, a street, a city and a state Customer Extension Street Customer Address City State --The Entity Relationship Model(1)--

  15. Attributes, cont. • Stored versus derived • E.g., an age can be derived from a stored birthdate • Additional constraints - not represented in a diagram • Null versus non-null • Domain constrained • E.g., a student number must be exactly 9 decimal digits DOB Customer Age --The Entity Relationship Model(1)--

  16. Keys in ER Diagrams • An attribute that is part of the (primary) key isunderlined. • E.g., ID uniquely identifies customers • Assume no two customers with the same name live at the same address. ID Name Customer Address ID Name Customer Address --The Entity Relationship Model(1)--

  17. Relationship Definition: A relationship is an object that associates entities. relationship objects Finding Nemo copy 1 Joe Jenkins Babe copy 3 Jane Doe DVD entities Customer entities --The Entity Relationship Model(1)--

  18. Relationship sets Definition: A relationship set is a specific set of associated objects between entities of the same entity types. {(e1, e2,...,en) | e1E1e2E2 .... enEn} where the relationship is (e1, e2,...,en). • Example is the set {(Joe Jenkins, Eraserhead copy 1), (Joe Jenkins, Babe copy 3)} Eraserhead copy 1 Joe Jenkins Babe copy 3 Jane Doe Video Tape entities Customer entities

  19. Relationship types • Definition: A relationship type describes a relationship set. • A relationship type is an ideal form (Plato), a kind, a type. • Example: TheRentsrelationship type relates the entity typeCustomerwith the entity typeDVD, capturing which customers currently rent DVDs. --The Entity Relationship Model(1)--

  20. Relationship Types - Diamond Notation • A relationship type is represented with a labelled diamond, and has lines to each of the entity types it relates. • The relationship type could have attributes. • A relationship type does not have key attribute(s) – the key comes from entity types that it relates. Customer ID Address Rents DVD Customer Name Location Return Date

  21. DVD Store Database Example • A DVDis an entity type with attributes • DVD Number, Film ID (the key attributes), and Status • A Customer is an entity type with attributes • Customer ID (the key), Name, Address • Rents is a relationship type with attributes • Return Date, Amount Paid Customer Rents DVD

  22. Example, cont. • The entity-relationship diagram for the above example is: Customer ID Amount Paid Status Name Rents Customer DVD Address DVD Number Film ID Return Date Where: Rents Customer Name EntityType Relationship Type Attribute

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