IT 5433 LM2 ER & EER Model

Learning Objectives: Explain importance of data modeling Define and use the entity-relationship model Define E/R terms Describe the enhanced E/R model Describe how super types and sub-types are modeled Explain when you would use an enhanced E/R model

What is Data Modeling? Data modeling is the act of exploring data-oriented structures. Data models can be used for a variety of purposes, from high-level conceptual models to physical data models. With data modeling you identify entity types whereas with class modeling you identify classes. Data attributes are assigned to entity types just as you would assign attributes and operations to classes. Traditional data modeling is different from class modeling because it focuses solely on data – class models allow you to explore both the behavior and data aspects of your domain, with a data model you can only explore data issues.

Three Basic Styles of Data Model: Conceptual - sometimes called domain models, are typically used to explore domain concepts with project stakeholders. Logical (LDM)- logical data model, used to explore the domain concepts, and their relationships, of your problem domain. Physical (PDM)- physical data model, used to design the internal schema of a database, depicting the data tables, the data columns of those tables, and the relationships between the tables.

Data Model: An entity type represents a collection of similar objects. Each entity type will have one or more data attributes.

Entity-Relationship (ER) Model E-R Model captures meanings rather than an actual implementation The E-R Model is viewed as set of: Entities Relationships among entities Symbols used in E-R Diagram Entity – (rectangle) an object that is involved in the enterprise and that be distinguished from other objects Attribute – (oval) describes one aspect of an entity type. Relationship – (diamond) Link – (line)

Entity-Relationship (ER) Model Keys Superkey: an attribute or set of attributes that uniquely identifies an entity Composite key: a key requiring more than one attribute Candidate key: a superkey such that no proper subset of its attributes is also a superkey (minimal superkey – has no unnecessary attributes) Primary key: the candidate key chosen to be used for identifying entities and accessing records. Unless otherwise noted "key" means "primary key" Alternate key: a candidate key not used for primary key Secondary key: attribute or set of attributes commonly used for accessing records, but not necessarily unique Foreign key: term used in relational databases (but not in the E-R model) for an attribute that is the primary key of another table and is used to establish a relationship with that table where it appears as an attribute also.

Enhanced Entity-Relationship (EER) Model One entity type might be a subtype of another. A relationship exists between a entity and the corresponding entity is called IsA. Type Hierarchy: Covering constraint: Union of subtype entities is equal to set of supertype entities. An entity is an element of at least one subtype Disjointness constraint: Sets of subtype entities are disjoint from one another (i.e., the sets are mutually exclusive). An entity can be an element of at most one entity

IsA Typically used in databases to be implemented as Object Oriented Models. The upper entity type is the more abstract entity type (supertype) from which the lower entities inherit its attributes

Super Types and Sub-Types Inheritance - All attributes of the supertype apply to the subtype. The subtype inherits all attributes of its supertype. The key of the supertype is also the key of the subtype Transitivity - This property creates a hierarchy of IsA relationships Example: Student is subtype of Person, Freshman is subtype of Student, therefore Freshman is also a subtype of Person Advantage: Used to create a more concise and readable E-R diagram. It best maps to object oriented approaches either to databases or related applications. Attributes common to different entity sets need not be repeated They can be grouped in one place as attributes of the supertype Attributes of (sibling) subtypes are likely to be different (and should be for this to be very useful)

ER Notations u=er_notations Mandatory- Optional (1:1 Binary)Mandatory-Optional (Complete, disjoint)

ER Notations u=er_notations Crow’s Feet (1-N Binary Identifying) Crow’s Feet (1-N Unary)

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