Entity Relationship (E-R) Modeling 10/15/2002 TCSS445A Isabelle Bichindaritz

TCSS445A Isabelle Bichindaritz Learning Objectives Conceptual model(s) Internal and external models Definition and refinement of relationships between entities during the database design process ERD components and database design and implementation Interpretation of the modeling symbols for the four most popular E-R modeling tools 10/15/2002 TCSS445A Isabelle Bichindaritz

Basic Modeling Concepts Art and science Good judgment coupled with powerful design tools Models “Description or analogy used to visualize something that cannot be directly observed” Webster’s Dictionary “A model is a representation of the world in simplified terms, it is an abstraction of the real world” Data Model Relatively simple representation of complex real-world data structures 10/15/2002 TCSS445A Isabelle Bichindaritz

Data Models: Degrees of Data Abstraction Figure 3.1 10/15/2002 TCSS445A Isabelle Bichindaritz

Degrees of Abstraction Conceptual Global view of data from application domain, based on end-users requirements Basis for identification and description of main data items ERD used to graphically represent conceptual data model (or class diagram in UML) Hardware and software (and DBMS) independent Internal Representation of database as seen by DBMS Adapts conceptual model to a specific DBMS Software dependent 10/15/2002 TCSS445A Isabelle Bichindaritz

Degrees of Abstraction External Users’ views of data environment Provides subsets of internal view 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 10/15/2002 TCSS445A Isabelle Bichindaritz

Degrees of Abstraction Three main levels of data models: deliverables Conceptual data model Project initiation and planning: ERD’s with entities and relationships only Analysis: ERD’s refined with attributes Logical data model = Internal + external data model: a set of normalized relations, based on ERD and views/forms design Physical data model = physical file and database design 10/15/2002 TCSS445A Isabelle Bichindaritz

Conceptual Data Model Example 10/15/2002 TCSS445A Isabelle Bichindaritz

Internal / External Data Models Example 10/15/2002 TCSS445A Isabelle Bichindaritz

The Entity Relationship (E-R) Model Represents conceptual view Main Components Entities Stands for entity set Corresponds to entire table, not row Represented by rectangle Rows correspond to entity instances or entity occurrences Attributes Represented by ovals or in entity Relationships Represented by diamonds or just a relationship name 10/15/2002 TCSS445A Isabelle Bichindaritz

TCSS445A Isabelle Bichindaritz Attributes Characteristics of entities Domain is set of possible values ( (true, false), … ) Primary keys underlined 10/15/2002 TCSS445A Isabelle Bichindaritz

TCSS445A Isabelle Bichindaritz Attributes 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 Can be represented by a 1-M relationship Derived Can be derived with algorithm Age can be derived from date of birth 10/15/2002 TCSS445A Isabelle Bichindaritz

TCSS445A Isabelle Bichindaritz Attributes Examples: CLASS(CLASS_CODE, CRS_CODE, CLASS_SECTION, CLASS_TIME, CLASS_ROOM, PROF_NUM) CLASS(CRS_CODE, CLASS_SECTION, CLASS_TIME, CLASS_ROOM, PROF_NUM) STUDENT(Student_Id, Student_Name, Address, Phone_Number, Major) 10/15/2002 TCSS445A Isabelle Bichindaritz

Multivalued Attributes 10/15/2002 TCSS445A Isabelle Bichindaritz

Multivalued Attributes 10/15/2002 TCSS445A Isabelle Bichindaritz

TCSS445A Isabelle Bichindaritz Derived Attributes 10/15/2002 TCSS445A Isabelle Bichindaritz

TCSS445A Isabelle Bichindaritz 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 associated with one occurrence of related entity: (1,4), (1,N), … How many classes does a professor teach ? (1,4) 10/15/2002 TCSS445A Isabelle Bichindaritz

Connectivity and Cardinality in an ERD Figure 3.12 10/15/2002 TCSS445A Isabelle Bichindaritz

Relationship Strength Existence dependence Entity’s existence depends on existence of related entities Existence-independent entities can exist apart from related entities EMPLOYEE claims DEPENDENT Weak (non-identifying) One entity is existence-independent on another PK of related entity doesn’t contain PK component of parent entity Strong (identifying) One entity is existence-dependent on another PK of related entity contains PK component of parent entity 10/15/2002 TCSS445A Isabelle Bichindaritz

Weak Relationship IE = Inversion Entity = a non-unique identifier for an entity 10/15/2002 TCSS445A Isabelle Bichindaritz

TCSS445A Isabelle Bichindaritz Strong Relationship 10/15/2002 TCSS445A Isabelle Bichindaritz

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 Mandatory Entity occurrence requires corresponding occurrence in related entity If no optionality symbol is shown on ERD, it is mandatory 10/15/2002 TCSS445A Isabelle Bichindaritz

Optional Participation 10/15/2002 TCSS445A Isabelle Bichindaritz

TCSS445A Isabelle Bichindaritz Weak Entity Existence-dependent on another entity Has primary key that is partially or totally derived from parent entity Figure 3.19 10/15/2002 TCSS445A Isabelle Bichindaritz

TCSS445A Isabelle Bichindaritz 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 10/15/2002 TCSS445A Isabelle Bichindaritz

Three Types of Relationships Figure 3.21 10/15/2002 TCSS445A Isabelle Bichindaritz

TCSS445A Isabelle Bichindaritz Composite Entities Used to ‘bridge’ between M:N relationships Bridge entities composed of primary keys of each entity needing connection Figure 3.30 10/15/2002 TCSS445A Isabelle Bichindaritz

Composite Entities (con’t.) Figure 3.31 10/15/2002 TCSS445A Isabelle Bichindaritz

Composite Entities (con’t.) 10/15/2002 TCSS445A Isabelle Bichindaritz

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 10/15/2002 TCSS445A Isabelle Bichindaritz

Generalization Hierarchy with Disjoint Subtypes 10/15/2002 TCSS445A Isabelle Bichindaritz

Generalization Hierarchy with Overlapping Subtypes Figure 3.35 10/15/2002 TCSS445A Isabelle Bichindaritz

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 10/15/2002 TCSS445A Isabelle Bichindaritz

Comparison of E-R Modeling Symbols Figure 3.36 10/15/2002 TCSS445A Isabelle Bichindaritz

Developing an E-R Diagram Iterative Process Step1: General narrative of organizational operations developed Step2: Basic E-R Model graphically depicted and reviewed Step3: Modifications made to incorporate newly discovered E-R components Repeat process until designers and users agree E-R Diagram complete 10/15/2002 TCSS445A Isabelle Bichindaritz

Supertype/Subtype Relationship in an ERD Figure 3.42 10/15/2002 TCSS445A Isabelle Bichindaritz

First ERD Segment Established Figure 3.43 10/15/2002 TCSS445A Isabelle Bichindaritz

Second and Third ERD Segments Established Figures 3.44 & 3.45 10/15/2002 TCSS445A Isabelle Bichindaritz

Fourth and Fifth ERD Segments Established Figures 3.46 & 3.47 10/15/2002 TCSS445A Isabelle Bichindaritz

Sixth and Seventh ERD Segments Established Figures 3.48 & 3.49 10/15/2002 TCSS445A Isabelle Bichindaritz

Eighth ERD Segment Established Figures 3.50 10/15/2002 TCSS445A Isabelle Bichindaritz

Ninth ERD Segment Established Figures 3.51 10/15/2002 TCSS445A Isabelle Bichindaritz

Components of E-R Model Table 3.2 10/15/2002 TCSS445A Isabelle Bichindaritz

TCSS445A Isabelle Bichindaritz Completed ERD Figure 3.52 10/15/2002 TCSS445A Isabelle Bichindaritz

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 10/15/2002 TCSS445A Isabelle Bichindaritz

Burger Inventory Example The Burger store wants to develop a new inventory system. Analysts have determined that the following data are required to represent the data needed by the inventory system: An INVOICE includes one or more INVOICE ITEMS, each of which corresponds to an INVENTORY ITEM. Obviously, an INVOICE ITEM cannot exist without an associated INVOICE, and over time, there will be zero to many receipts, or INVOICE ITEMs, for an INVENTORY SYSTEM. 10/15/2002 TCSS445A Isabelle Bichindaritz

Burger Inventory Example Each PRODUCT has a RECIPE of INVENTORY ITEMs, containing several RECIPE LINEs. Thus, RECIPE LINE is an associative entity supporting a bill-of-materials type relationship between PRODUCT and INVENTORY ITEM. A SALE indicates that Burger sells one or more ITEM SALES, each of which corresponds to a PRODUCT. ITEM SALE cannot exist without an associated SALE, and over time there will be zero to many ITEM SALES for a PRODUCT. Note: the following ERD does not represent weak entities,and relationships. Do you see any ? 10/15/2002 TCSS445A Isabelle Bichindaritz

Burger Inventory Example 10/15/2002 TCSS445A Isabelle Bichindaritz