--The Entity Relationship Model(1)--1 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 conceptual design physical design database requirements conceptual schema logical schema internal schema data model mapping DBMS- specific DBMS- independent

--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-vision STORE. 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,  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  CUSTOMER entity examples Name=Jane, DOB=1/1/70 Name=Joe, DOB=5/6/75

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 Name=Jane, DOB=1/1/70 CUSTOMER entities CUSTOMER entity sets 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. Name=Jane, CUSTOMER entities unknown date of birth 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 one Customer with the Name ‘Joe’ and a DOB of 5/6/75.  Is there a smaller such set of attributes? Name=Joe, DOB=10/7/49 CUSTOMER entities Name=Fred, DOB=5/6/75 Name=Joe, DOB=5/6/75

--The Entity Relationship Model(1)--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 each Customer has a unique ID. 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 of Customer.  {ID} is a candidate key of Customer. 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 of Customer.

--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 Name DOB

--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 CustomerExtension Address Street City State Customer

--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 Customer DOB Age

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

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

18 Relationship sets Definition: A relationship set is a specific set of associated objects between entities of the same entity types. {(e 1, e 2,..., e n ) | e 1  E 1  e 2  E 2 ....  e n  E n } where the relationship is (e 1, e 2,..., e n ).  Example is the set {(Joe Jenkins, Eraserhead copy 1), (Joe Jenkins, Babe copy 3)} Joe Jenkins Jane Doe Eraserhead copy 1 Babe copy 3 Customer entities Video Tape entities

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

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 Rents DVD Name Customer ID Return Date Location Address

21 DVD Store Database Example  A DVD is 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 CustomerRentsDVD

22 Example, cont.  The entity-relationship diagram for the above example is: Name Customer Rents DVD Name Address Customer ID Return Date Amount PaidStatus DVD Number Film ID Customer Rents Entity TypeAttributeRelationship Type Where: