A l Ain University Of Science and Technology

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1 A l Ain University Of Science and Technology 4 Handout(4) Database Management Principles and Applications The Entity Relationship (ER) Model 1

2 In this chapter, you will learn: The main characteristics of entity relationship components How relationships between entities are defined and refined and how those relationships are incorporated into the database design process How ERD components affect database design and implementation That real-world database design often requires the reconciliation of conflicting goals 2

3 Data Modeling Using the Entity-Relationship (ER) Model Entity-Relationship (ER) model Popular high-level conceptual data model ER diagrams Diagrammatic notation associated with the ER model Unified Modeling Language (UML) 3

4 Using High-Level Conceptual Data Models for Database Design 4 Requirements collection and analysis Database designers interview prospective database users to understand and document data requirements Result: data requirements Functional requirements of the application: These consist of the userdefined operations (or transactions) that will be applied to the database, including both retrievals and updates 4

5 Database design methodology has 3 main phases: 1. Conceptual database design 2. Logical database design 3. Physical database design 5

6 Using High-Level Conceptual Data Models (cont d.) Conceptual schema Conceptual design (ER Diagram) Description of data requirements Includes detailed descriptions of the entity types, relationships, and constraints Transformed from high-level data model into implementation data model 6

7 Using High-Level Conceptual Data Models (cont d.) Logical design or data model mapping Result is a database schema in implementation data model of DBMS Table Generation From ER Model Normalization of Tables Physical design phase Internal storage structures, file organizations, indexes, access paths, and physical design parameters for the database files specified 7

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9 The Entity Relationship (ER) Model ER model forms the basis of an ER diagram ERD represents conceptual database as viewed by end user ERDs depict database s main components: Entities Attributes Relationships 9

10 Entities Refers to entity set and not to single entity occurrence Corresponds to table and not to row in relational environment In both Chen and Crow s Foot models, entity is represented by rectangle containing entity s name Entity name, a noun, is usually written in capital letters 10

11 Entities (continued) 11

12 Entity Types, Entity Sets, Keys, and Value Sets Entity type Collection (or set) of entities that have the same attributes 4 12

13 Entity Types, Entity Sets, Keys, and Value Sets (cont d.) Key or uniqueness constraint Attributes whose values are distinct for each individual entity in entity set Key attribute Uniqueness property must hold for every entity set of the entity type Value sets (or domain of values) Specifies set of values that may be assigned to that attribute for each individual entity 4 13

14 Attributes Characteristics of entities In Chen model, attributes are represented by ovals and are connected to entity rectangle with a line Each oval contains the name of attribute it represents In Crow s Foot model, attributes are written in attribute box below entity rectangle 14

15 Attributes (continued) Type of attributes: Simple (atomic) attributes Composite attributes Multivalued attributes Derived attributes NULL values Complex attributes 15

16 Attributes (continued) 16

17 Attributes (continued) 17

18 Identifiers (Primary Keys) Underlined in the ERD Key attributes are also underlined in frequently used table structure shorthand 18

19 Relationships Association between entities Participants are entities that participate in a relationship Relationships between entities always operate in both directions Relationship can be classified as 1:M Relationship classification is difficult to establish if know only one side of the relationship 19

20 The relationship attends is shown as a Diamond connection between the entities. 20

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22 In the previous example, the connecting lines terminated with 1, M, N 22

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27 In this diagram, the solid line signifies that a teacher may optionally be employed by a school. This is indicated by the double line attached to school. 27

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29 Constraints on Binary Relationship 4 Types (Cont.) Participation constraint Specifies whether existence of entity depends on its being related to another entity Types: total and partial Partial participation: only some members of the entity set are required to participate in a particular relationship in which that entity type is involved. Total participation: this happens when every entity of the entity set participates in a particular relationship. Total participation is also called existence dependency 29

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33 Weak Entities Weak entity meets two conditions Existence-dependent Cannot exist without entity with which it has a relationship Has primary key that is partially or totally derived from parent entity in relationship Database designer usually determines whether an entity can be described as weak based on business rules 33

34 Weak Entity Types (cont.) 4 Example: Consider the 1:N relationship HAS-DEPENDENT between EMPLOYEE : DEPENDENT which is used to indicate the dependents of an employee. The attributes of the entity DEPENDENT are: Dependent name, Birth date, Gender, Relationship to employee A weak entity type has a partial key, which is a set of attributes that can uniquely identify weak entity belonging to the same owner entity. In the above example, the attribute Dependent name is the partial key. 34

35 Weak Entities (continued) 4 35

36 Weak Entities (continued) 36

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42 Developing an ER Diagram Database design is iterative rather than linear or sequential process Iterative process Based on repetition of processes and procedures 42

43 Developing an ER Diagram (continued) Building an ERD usually involves the following activities: Create detailed narrative of organization s description of operations Identify business rules based on description of operations Identify main entities and relationships from business rules Develop initial ERD Identify attributes and primary keys that adequately describe entities Revise and review ERD 43

44 Summary Entity relationship (ER) model Uses ERD to represent conceptual database as viewed by end user ERM s main components: Entities Relationships Attributes Includes connectivity and cardinality notations 44

45 Summary (continued) Connectivities and cardinalities are based on business rules In ERM, M:N relationship is valid at conceptual level ERDs may be based on many different ERMs Database designers are often forced to make design compromises 45

46 ER Diagrams notations (summery) 46

A l Ain University Of Science and Technology

A l Ain University Of Science and Technology 4 Handout(4) Database Management Principles and Applications The Entity Relationship (ER) Model http://alainauh.webs.com/ http://www.comp.nus.edu.sg/~lingt