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3 Chapter 3 Entity Relationship (E-R) Modeling Hachim Haddouti.

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Presentation on theme: "3 Chapter 3 Entity Relationship (E-R) Modeling Hachim Haddouti."— Presentation transcript:

1 3 Chapter 3 Entity Relationship (E-R) Modeling Hachim Haddouti

2 3 Hachim Haddouti, CH3, see also Rob & Coronel 2 In this chapter, you will learn: What a conceptual model is and what its purpose is The difference between internal and external models How internal and external models serve the database design process How relationships between entities are defined and refined, and how such relationships are incorporated into the database design process How ERD components affect database design and implementation How to interpret the modeling symbols for the four most popular E-R modeling tools That real-world database design often requires you to reconcile conflicting goals

3 3 Hachim Haddouti, CH3, see also Rob & Coronel 3 Why Data Modeling Database designers, programmers and end users have different views of the data Data modeling abstracts real-world complexities to facilitate communication between designers, programmers and end users A data model is a relatively simple, usually graphic representation of real-world data structures, their characteristics, constraints, relations and transformations It is necessary for good database design and, hence, good applications

4 3 Hachim Haddouti, CH3, see also Rob & Coronel 4 Data Models: Degrees of Data Abstraction

5 3 Hachim Haddouti, CH3, see also Rob & Coronel 5 Degrees of Abstraction Conceptual –Global view of data –Basis for identification and description of main data items –ERD used to represent conceptual data model –Hardware and software independent Internal –Representation of database as seen by DBMS –Adapts conceptual model to specific DBMS –Software dependent

6 3 Hachim Haddouti, CH3, see also Rob & Coronel 6 Degrees of Abstraction (con’t.) External –Users’ views of data environment (functional modules) –Provides subsets of internal view (student registration, class scheduling) –Makes application program development easier –Facilitates designers’ tasks –Ensures adequacy of conceptual model –Ensures security constraints in design Physical –Lowest level of abstraction –Software and hardware dependent –Requires definition of physical storage devices and access methods

7 3 Hachim Haddouti, CH3, see also Rob & Coronel 7 The Entity Relationship (E-R) Model E-R Model: graphical data representation Represents conceptual view Main Components –Entities (real world objects) Corresponds to entire table, not row Represented by rectangle E.g. PERSON, PLACE, BOOK, CAR, EMPLOYEE –An entity set is a set of entities of the same type that share the same properties. E.g: set of all persons, companies, trees, holidays –Attributes –Relationships Complementary of Relational Model ( an entity corresponds to a relational table, an entity occurrence to a row)

8 3 Hachim Haddouti, CH3, see also Rob & Coronel 8 Attributes Characteristics of entities Domain is set of possible values, e.g. GPA(0,4) Primary keys underlined

9 3 Hachim Haddouti, CH3, see also Rob & Coronel 9 Attributes (con’t.) Simple –Cannot be subdivided –Age, sex, marital status Composite –Can be subdivided into additional attributes –Address into street, city, zip Single-valued –Can have only a single value –Person has one social security number Multi-valued –Can have many values –Person may have several college degrees, phone numbers –Implementation!!! Derived –Can be derived with algorithm –Age can be derived from date of birth

10 3 Hachim Haddouti, CH3, see also Rob & Coronel 10 Relationships Association between entities Connected entities are called participants Operate in both directions Connectivity describes relationship classification –1:1, 1:M, M:N Cardinality –Expresses number of entity occurrences/instances associated with one occurrence of related entity

11 3 Hachim Haddouti, CH3, see also Rob & Coronel 11 Connectivity and Cardinality in an ERD (business rules)

12 3 Hachim Haddouti, CH3, see also Rob & Coronel 12 Relationship Strength Existence dependence –Entity’s existence depends on existence of related entities –EMPLOYEE claims DEPENDENT Weak relationship (non-identifying) –One entity is existence-independent on another –PK of related entity doesn’t contain PK component of parent entity Strong relationship (identifying) –One entity is existence-dependent on another –PK of related entity contains PK component of parent entity

13 3 Hachim Haddouti, CH3, see also Rob & Coronel 13 Relationship Participation Optional –Entity occurrence does not require a corresponding occurrence in related entity –Shown by drawing a small circle on side of optional entity on ERD –E.g. INVOICE is optional to CUSTOMER, or INVOICE and PRODUCT Mandatory –Entity occurrence requires corresponding occurrence in related entity –If no optionality symbol is shown on ERD, it is mandatory

14 3 Hachim Haddouti, CH3, see also Rob & Coronel 14 Example of Optional Entitites

15 3 Hachim Haddouti, CH3, see also Rob & Coronel 15 Weak Entity Existence-dependent on another entity Has primary key that is partially or totally derived from parent entity

16 3 Hachim Haddouti, CH3, see also Rob & Coronel 16 Relationship Degree Indicates number of associated entities Unary –Single entity –Recursive –Exists between occurrences of same entity set Binary –Two entities associated Ternary –Three entities associated

17 3 Hachim Haddouti, CH3, see also Rob & Coronel 17 Three Types of Relationships

18 3 Hachim Haddouti, CH3, see also Rob & Coronel 18 Composite Entities Used to ‘bridge’ between M:N relationships Bridge entities composed of primary keys of each entity needing connection

19 3 Hachim Haddouti, CH3, see also Rob & Coronel 19 Bridge Entity

20 3 Hachim Haddouti, CH3, see also Rob & Coronel 20 Entity Supertypes and Subtypes Generalization hierarchy –Depicts relationships between higher-level supertype and lower-level subtype entities –Supertype has shared attributes –Subtypes have unique attributes –Disjoint relationships Unique subtypes Non-overlapping Indicated with a ‘G’ –Overlapping subtypes use ‘Gs’ Symbol

21 3 Hachim Haddouti, CH3, see also Rob & Coronel 21 Generalization Hierarchy with Overlapping Subtypes

22 3 Hachim Haddouti, CH3, see also Rob & Coronel 22 Comparison of E-R Modeling Symbols Alternate styles developed to enable easier use of CASE tools Chen –Moved conceptual design into practical database design arena Crow’s Foot –Cannot detail all cardinalities Rein85 –Similar to Crow’s Foot –Operates at higher level of abstraction IDEF1X –Derivative of ICAM studies in the late 1970’s –Uses fewer symbols

23 3 Hachim Haddouti, CH3, see also Rob & Coronel 23 Comparison of E-R Modeling Symbols

24 3 Hachim Haddouti, CH3, see also Rob & Coronel 24 Challenge of Database Design: Conflicting Goals Database must be designed to conform to design standards High-speed processing may require design compromises Quest for timely information may be the focus of database design Other concerns –Security –Performance –Shared access –Integrity

25 3 Hachim Haddouti, CH3, see also Rob & Coronel 25 Entity-Relationship Model Example of schema in the entity-relationship model

26 3 Hachim Haddouti, CH3, see also Rob & Coronel 26 A Sample Relational Database

27 3 Hachim Haddouti, CH3, see also Rob & Coronel 27 E-R Diagram for Insurance

28 3 Hachim Haddouti, CH3, see also Rob & Coronel 28 Draw ERD for Ifrane School The Ifrane School (IS) has contacted you to create a conceptual model whose application will meet the expected database requirements for its training program. Those are their business rules: IS has 12 instructors and can handle up to 30 trainees per class. IS offers five "advanced technology" courses, each of which may generate several classes. If a class has fewer than 10 trainees in it, it will be canceled. It is, therefore, possible for a course not to generate any classes during a session. Each class is taught by one instructor. Each instructor may teach up to two classes or may be assigned to do research. Each trainee may take up to two classes per session.

29 3 Hachim Haddouti, CH3, see also Rob & Coronel 29 Interpretation of IS biz Rules Basically, 3 sets of relationships exist: –A COURSE may generate one or more CLASSes, –an INSTRUCTOR teaches up to two CLASSes, –and a TRAINEE may enroll in up to two CLASSes. A trainee can take more than one class, and each class contains many (10 or more) trainees, so there is a M:N relationship between TRAINEE and CLASS. A class is taught by only one instructor, but an instructor can teach up to two classes. Therefore, there is a 1:M relationship between INSTRUCTOR and CLASS. Finally, a COURSE may generate more than one CLASS, while each CLASS is based on one COURSE, so there is a 1:M relationship between COURSE and CLASS. Note the optional and mandatory relationships: –a CLASS must have TRAINEEs enrolled in it, but TRAINEEs do not necessarily take CLASSes. (Some may take "on the job training.") –An INSTRUCTOR may not be teaching any CLASSes, doing research instead, but each CLASS must have an INSTRUCTOR. If not enough people sign up for a CLASS, a COURSE may not generate any CLASSes, but each CLASS must represent a COURSE.

30 3 Hachim Haddouti, CH3, see also Rob & Coronel 30 ERD for Ifrane School


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