Entity Relationship ER Modeling 10152002 TCSS 445 A

Entity Relationship (E-R) Modeling 10/15/2002 TCSS 445 A Isabelle Bichindaritz 1

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 TCSS 445 A Isabelle Bichindaritz 2

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 TCSS 445 A Isabelle Bichindaritz 3

Data Models: Degrees of Data Abstraction Figure 3. 1 10/15/2002 TCSS 445 A Isabelle Bichindaritz 4

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 TCSS 445 A Isabelle Bichindaritz 5

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 TCSS 445 A Isabelle Bichindaritz 6

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 TCSS 445 A Isabelle Bichindaritz 7

Conceptual Data Model Example 10/15/2002 TCSS 445 A Isabelle Bichindaritz 8

Internal / External Data Models Example 10/15/2002 TCSS 445 A Isabelle Bichindaritz 9

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 TCSS 445 A Isabelle Bichindaritz 10

Attributes • Characteristics of entities • Domain is set of possible values ( (true, false), … ) • Primary keys underlined 10/15/2002 TCSS 445 A Isabelle Bichindaritz 11

Attributes • Simple • Multi-valued – 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 10/15/2002 – 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 TCSS 445 A Isabelle Bichindaritz 12

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 TCSS 445 A Isabelle Bichindaritz 13

Multivalued Attributes 10/15/2002 TCSS 445 A Isabelle Bichindaritz 14

Multivalued Attributes 10/15/2002 TCSS 445 A Isabelle Bichindaritz 15

Derived Attributes 10/15/2002 TCSS 445 A Isabelle Bichindaritz 16

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 TCSS 445 A Isabelle Bichindaritz 17

Connectivity and Cardinality in an ERD Figure 3. 12 10/15/2002 TCSS 445 A Isabelle Bichindaritz 18

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 TCSS 445 A Isabelle Bichindaritz 19

Weak Relationship IE = Inversion Entity = a non-unique identifier for an entity 10/15/2002 TCSS 445 A Isabelle Bichindaritz 20

Strong Relationship 10/15/2002 TCSS 445 A Isabelle Bichindaritz 21

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 TCSS 445 A Isabelle Bichindaritz 22

Optional Participation 10/15/2002 TCSS 445 A Isabelle Bichindaritz 23

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 TCSS 445 A Isabelle Bichindaritz 24

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 TCSS 445 A Isabelle Bichindaritz 25

Three Types of Relationships Figure 3. 21 10/15/2002 TCSS 445 A Isabelle Bichindaritz 26

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 TCSS 445 A Isabelle Bichindaritz 27

Composite Entities (con’t. ) Figure 3. 31 10/15/2002 TCSS 445 A Isabelle Bichindaritz 28

Composite Entities (con’t. ) 10/15/2002 TCSS 445 A Isabelle Bichindaritz 29

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 TCSS 445 A Isabelle Bichindaritz 30

Generalization Hierarchy with Disjoint Subtypes 10/15/2002 TCSS 445 A Isabelle Bichindaritz 31

Generalization Hierarchy with Overlapping Subtypes Figure 3. 35 10/15/2002 TCSS 445 A Isabelle Bichindaritz 32

Comparison of E-R Modeling • Alternate styles Symbols developed to enable easier use of CASE tools • Chen – Moved conceptual design into practical database design arena • Crow’s Foot – Cannot detail all cardinalities • Rein 85 – Similar to Crow’s Foot – Operates at higher level of abstraction • IDEF 1 X – Derivative of. TCSS 445 A ICAM Isabelle studies in the late 1970’s Bichindaritz – Uses fewer symbols 10/15/2002 33

Comparison of E-R Modeling Symbols Figure 3. 36 10/15/2002 TCSS 445 A Isabelle Bichindaritz 34

Developing an E-R Diagram • Iterative Process – Step 1: General narrative of organizational operations developed – Step 2: Basic E-R Model graphically depicted and reviewed – Step 3: Modifications made to incorporate newly discovered E-R components • Repeat process until designers and users agree E-R Diagram complete 10/15/2002 TCSS 445 A Isabelle Bichindaritz 35

Supertype/Subtype Relationship in an ERD Figure 3. 42 10/15/2002 TCSS 445 A Isabelle Bichindaritz 36

First ERD Segment Established Figure 3. 43 10/15/2002 TCSS 445 A Isabelle Bichindaritz 37

Second and Third ERD Segments Established Figures 3. 44 & 3. 45 10/15/2002 TCSS 445 A Isabelle Bichindaritz 38

Fourth and Fifth ERD Segments Established Figures 3. 46 & 3. 47 10/15/2002 TCSS 445 A Isabelle Bichindaritz 39

Sixth and Seventh ERD Segments Established Figures 3. 48 & 3. 49 10/15/2002 TCSS 445 A Isabelle Bichindaritz 40

Eighth ERD Segment Established Figures 3. 50 10/15/2002 TCSS 445 A Isabelle Bichindaritz 41

Ninth ERD Segment Established Figures 3. 51 10/15/2002 TCSS 445 A Isabelle Bichindaritz 42

Components of E-R Model Table 3. 2 10/15/2002 TCSS 445 A Isabelle Bichindaritz 43

Completed ERD Figure 3. 52 10/15/2002 TCSS 445 A Isabelle Bichindaritz 44

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 TCSS 445 A Isabelle Bichindaritz 45

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 TCSS 445 A Isabelle Bichindaritz 46

Burger Inventory Example – Each PRODUCT has a RECIPE of INVENTORY ITEMs, containing several RECIPE LINEs. Thus, RECIPE LINE is an associative entity supporting a billof-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 TCSS 445 A Isabelle Bichindaritz 47

Burger Inventory Example 10/15/2002 TCSS 445 A Isabelle Bichindaritz 48