Conceptual Data Modeling The Entity-Relationship Diagram. Bridging the Gap

Transcription

1 Chapter Conceptal Data Modeling The Entity-Relationship Diagram Conceptal Perception of an Enterprise Bridging the Gap STUDENT( ID, Name, Age, Address, GPA ) INSTRUCTOR ( Emp#, Name, Rank, Dept ) COURSE ( Corse#, Credits, Title ) CLASS ( Emp#, ID, Corse#, Time, Room ) 1

2 What Is Conceptal Data Modeling? A process that t represents e e the entities, relationships, and activities of an enterprise in terms of a set of abstract concepts of a chosen data model for specific prposes. Enterprise Modeling, Bsiness Modeling Data Modeling What is Conceptal Data Modeling Entity-Relationship (E-R ) Modeling Limitations of E-R Modeling 2

3 Types of Information Static Information Data -- Entities Associations -- Relationships among entities Dynamic Information (Not covered) Activities -- Operations/transactions Integrity constraints -- Bsiness rles/reglations and data meanings Entity-Relationship Modeling Basic modeling concepts: Entities, entity types, and attribtes Relationships Date Works_for 1 Department Langage Emp# N Instrctor Date 1 1 Assigned Office FName Name MInit LName N Teaches M Time Location Corse 3

4 E-R Notation Relationship or Entity Type Relationship Attribte Primary Key Entities An entity is a conceptal object Physically exists Usally a non in reqirement specification Jose Alice Steve CIS 2010 CIS 3730 Class Stdent Acct CIS Department 4

5 Entity Types A collection of similar entities An abstraction of "physical" entities A non in reqirement specifications Having "independent" meaning Stdent Department Jose Steve Alice Corse CIS Acct CIS 2010 CIS 3730 Attribtes Properties or characteristics of entities and entity types Attribte vales -- Properties of entities Vale set - All acceptable attribte vales Attribtes (definitions) -- Properties of entity types A non or an adjective in reqirement specifications No "independent" d meaning Stdent ID Age Jose " " 25 5

6 Key Attribtes One or a grop of attribtes that can niqely identify individal entities of an entity type A key refers to one or a grop of attribtes as a whole A key attribte is a component attribte of a key Key changes with data semantics An entity type may have several qalified keys Primary key -- One of the candidate keys Alternate key - Candidate keys not sed as the primary key Secondary key -- An identifier of records with similar properties of interest The primary key attribte(s) is(are) nderlined Relationships Associations among entities Relationships -- Associations among entities Usally a verb in reqirement specification Stdent Stdent Takes or Takes Corse Corse 6

7 Relationship Degrees The nmber of entity types associated with that relationship. Unary Employee Spervises Binary Employee Work Department Ternary n-ary Project Spply Part Spplier Relationship Cardinality How entities are connected throgh a relationship One-to-One -- An entity of E1 is connected to at most one entity of E2 and vice versa. E1 a b c R 1 1 [] E2 x y z One-to-Many -- An entity of E1 may be connected to one or more entities of E2, bt an entity of E2 can only be mapped to at most one entity of E1. a b c 1 M x y z Many-to-Many -- An entity of E1 may be linked to one or more entities of E2, and vice versa. a b c M N x y z 7

8 Combined Notation one-to-one one-to-many many-to-many Other E-R Models Relationship Entity 1 Entity 2 attr. 1 attr. 2 Entity 1 Relationship Entity 2 ( 1:m ) ( 0:5 ) Entity 1 Relationship Entity 2 8

9 Pros and Cons of E-R Advantages Simple and easy to nderstand. Very poplar. Semantic richer than classical data models. Disadvantages: Not a formally defined data model. Deals with some integrity it constraints. Difficlt to distingish entities from relationships. Has redndant modeling information. Emp#, Name, Address Salary, Skill Project Member m-has Dependent has Project Manager ID, Name Address Birth-date Works-on Project Manages Emp#, Name, Address Salary, Skill 9