E-R Diagram. Bagian I Entity Concept

Size: px

Start display at page:

Download "E-R Diagram. Bagian I Entity Concept"

Ernest Lucas
6 years ago
Views:

1 E-R Diagram Bagian I Entity Concept

2 Database Desain Conceptual perspective User s perspective Database Requirements The Entity Relationalship (ER) Model is one of the most widely used mthod for conceptual design Conceptual Design Logical Design (Mapping) Conceptual Schema (ER) The Relational Model is the basic for several commercial DBMSs Logical Schema (Relational) Internal Schema

Phases of Database Design Universe of Discourse First step of the design process is to identify the Universe of Discourse (UoD) The database to be built will not

3 Phases of Database Design Universe of Discourse First step of the design process is to identify the Universe of Discourse (UoD) The database to be built will not model everything in the world, but rather some mini-world or Universe of Discourse. The UoD is the relevant portion of the real world to be modeled by the database 3

4 Phases of Database Design Universe of Discourse REQUIREMENTS COLLECTION AND ANALYSIS FUNCTIONAL ANALYSIS APPLICATION PROGRAM. Functional Analysis consists of specifying operations (retrievals, updates) that will be applied to the database Various techniques for software design exist, such as sequence diagrams, flowcharts, DFDs, etc. Refer to Software Engineering Texts 4

5 Phases of Database Design Universe of Discourse REQUIREMENTS COLLECTION AND ANALYSIS FUNCTIONAL ANALYSIS APPLICATION PROGRAM... CONCEPTUAL Description of the data requirements of users Expressed using a high level model such as ENTITY-RELATIONSHIP (ER) A database schema in the ER Model can be represented pictorially (ER diagrams) ER Model contains detailed descriptions of: What are the entities and relationships in the enterprise? What information about these entities and relationships should we store in the database? What are the integrity constraints or business rules that hold? 5

6 Phases of Database Design Universe of Discourse REQUIREMENTS COLLECTION AND ANALYSIS FUNCTIONAL ANALYSIS APPLICATION PROGRAM. CONCEPTUAL LOGICAL Actual implementation of the database, using a commercial DBMS Depends on the logical/implementation model of the DBMS, for example Relational Database Model, Object Oriented Database Model. Conceptual Model (ER) is mapped to the implementation model (Relational) in this phase 6

7 Phases of Database Design Database is tuned for storage and performance Includes specification of storage structures, access paths, file organization etc. Universe of Discourse REQUIREMENTS COLLECTION AND ANALYSIS FUNCTIONAL ANALYSIS APPLICATION PROGRAM... CONCEPTUAL LOGICAL PHYSICAL 7

8 Phases of Database Design Universe of Discourse REQUIREMENTS COLLECTION AND ANALYSIS FUNCTIONAL ANALYSIS APPLICATION PROGRAM. CONCEPTUAL LOGICAL PHYSICAL Data Structures and Operations are closely linked Design of the two is done in parallel and subsequent implementation in interdependent 8

9 Conceptual Design Universe of Discourse REQUIREMENTS COLLECTION AND ANALYSIS CONCEPTUAL Database Requirements Defines the UoD, and captures semantics & constraints of the UoD FUNCTIONAL ANALYSIS LOGICAL CONCEPTUAL Focus of this lecture APPLICATION PROGRAM. PHYSICAL Conceptual Schema (High Level Data Model, e.g. ER, NIAM, SDM) Serves as a medium for communicating the UoD 9

10 Contents The Entity-Relationship Model Conceptual Design and how conceptual design relates to the database design process Entity-Relationship (ER) Conceptual Design Method Notation Guide Example Illustrations 10

11 ER Model Basics Entities Entities, Entity Sets and Entity Types Attributes, Keys and Value Sets Relationships Relationship Types and Sets Relationship Degree Roles and Recursive Relationships Relationship Constraints Attributes of Relationship Types 11

12 ER Diagram Basics Entity sname Store Locations manager Relationship qty Keeps pname Attributes Product price descrip 12

13 13 PROJECT EMPLOYEE Fname Minit Name Lname Bdate Ssn Sex Salary Address DEPENDENT Name Number DEPARTMENT MANAGES WORKS_FOR WORK_ON CONTROLS SUPERVISION N Relationship Name BirthDate Sex Name Number Location Number Of Employee StartDate supervisor supervise 1 N N N M N Hours Example ER Model: Company Database DEPENDENTS_OF Locations

14 Entities Entity Entity Sets Entity Type Attributes (Types, Keys and Value Sets) 14

15 Real-world object distinguishable from other objects (e.g a student, car, job, subject, building...) An entity is described using a set of attributes The same entity may have different prominence in different UoDs In the Company database, an employee s car is of lesser importance In the Department of Transportation s registration database, cars may be the most important concept In both cases, cars will be represented as entities; but with different levels of detail Entity 15

16 Entity Sets A collection of similar entities (e.g. all employees) All entities in an entity set have the same set of attributes Each entity set has a key Each attribute has a domain Can map entity set to a relation easily EMPLOYEES SSN NAME SAL Kim 23, Jones 45,000 CREATE TABLE Employees (ssn CHAR(11), name CHAR(20), sal INTEGER, PRIMARY KEY (ssn)) 16

17 Entity Type Defines set of entities that have the same attributes (e.g. EMPLOYEE) Each Entity Type is described by its NAME and attributes The Entity Type describes the Schema or Intension for a set of entities Collection of all entities of a particular entity type at a given point in time is called the Entity Set or Extension of an Entity Type Entity Type and Entity Set are customarily referred to by the same name SSN name EMPLOYEE Notation sal 17

18 Attributes Key Attributes Value Sets of Attributes Null Valued Attributes Attribute Types Composite Vs. Simple Attributes Single-valued Vs. Multi-valued Attributes Derived Vs. Stored Attributes Notation 18

19 Key Attributes Key (or uniqueness) constraints are applied to entity types Key attribute s values are distinct for each individual entity in the entity set A key attribute has its name underlined inside the oval Key must hold for every possible extension of the entity type Multiple keys are possible SSN EMPLOYEE 19

20 Value Sets of Attributes Value sets specify the set of values that may be assigned to a particular attribute of an entity Employee Age: Integers between 21 & 65 Vehicle Registration Number: String of 3 alphabets followed by 3 integers Value sets map to relational domains Value sets are not displayed on the ER diagram 20

21 Null Valued Attributes A particular entity may not have an applicable value for an attribute Tertiary-Degree: Not applicable for a person with no university education Home-Phone: Not known if it exists Height: Not known at present time Type of Null Values Not Applicable Unknown Missing 21

22 Composite Vs. Simple Attributes Composite attributes can be divided into smaller parts which represent simple attributes with independent meaning Simple Attribute: Aircraft-Type Complex Attribute: Aircraft-Location which is comprised of : Aircraft-Latitude Aircraft-Longitude Aircraft-Altitude There is no formal concept of composite attribute in the relational model Notation 22

23 Single Vs. Multivalued Attributes Simple attributes can either be single-valued or multi-valued Single-valued: Gender = F Notation Multivalued: Degree = {BSc, MInfTech} Notation An attribute in the relational model is always single valued - Values are atomic! 23

24 Derived Vs. Stored Attributes Some attribute values can be derived from related attribute values: Age Date - B-day Y-Sal 12 * M-Sal Notation B-days Age M-sal Y-sal EMPLOYEE 24

25 Derived Vs. Stored Attributes Some attribute values can be derived from attributed values of related entities total-value sum (qty * price) Order Item Total-Value qty price 25

26 Representing Attributes Parenthesis ( ) for composite attributes Brackets { } for multi-valued attributes Assume a person can have more than one residence and each residence can have multiple telephones {AddressPhone ({ Phone ( AreaCode,PhoneNum ) }, Address (StreetAddresss (Number, Street, AptNo), City,State,PostalCode) ) } 26

27 An attribute A is defined as function A A:E _P(V) where E is the Entity Type V is the Value Set P(V) is the Power Set (set of all subsets) of V Formally... Value of attribute A for entity e is A(e) where e is an entity of type E 27

28 Formally... A is a Simple Attribute where A(e) is A singleton (set with one element) for single-valued attributes A set with multiple elements for multi-valued attributes An empty set for null valued attributes A is a Composite Attribute when the value set V is the Cartesian product of sets P(V1), P(V2),...,P(Vn) where V1,V2,...,Vn are value sets for the simple component attributes that form A: V = P(V1) X P(V2) X... X P(Vn) 28

29 Entities Entities, Entity Sets and Entity Types Attributes, Keys and Value Sets Relationships Relationship Types and Sets Relationship Degree Roles and Recursive Relationships Relationship Constraints Attributes of Relationship Types ER Model Basics 29

Data Modeling with the Entity Relationship Model. CS157A Chris Pollett Sept. 7, 2005.

Data Modeling with the Entity Relationship Model CS157A Chris Pollett Sept. 7, 2005. Outline Conceptual Data Models and Database Design An Example Application Entity Types, Sets, Attributes and Keys Relationship