Fundamentals of Database Systems

Fundamentals of Database Systems

METU Department of Computer Eng Ceng 302 Introduction to DBMS Entity-Relationship (ER) Model by Pinar Senkul resources: mostly froom Elmasri, Navathe and other books Chapter Outline Example Database Application (COMPANY) ER Model Concepts Entities and Attributes Entity Types, Value Sets, and Key Attributes Relationships and Relationship Types Weak Entity Types

Roles and Attributes in Relationship Types ER Diagrams - Notation ER Diagram for COMPANY Schema Example COMPANY Database Requirements of the Company (oversimplified for illustrative purposes) The company is organized into DEPARTMENTs. Each department has a name, number and an employee who manages the department. We keep track of the start date of the department manager. Each department controls a number of PROJECTs. Each project has a name,

number and is located at a single location. Example COMPANY Database (Cont.) We store each EMPLOYEEs TC id no, address, salary, sex, and birthdate. Each employee works for one department but may work on several projects. We keep track of the number of hours per week that an employee currently works on each project. We also keep track of the direct supervisor of each employee. Each employee may have a number of DEPENDENTs. For each dependent, we keep track of their name, sex, birthdate, and relationship to employee.

ER Model Concepts Entities and Attributes Entities are specific objects or things in the mini-world that are represented in the database. For example the EMPLOYEE Hulya Avsar, the Research DEPARTMENT, the GIS PROJECT Attributes are properties used to describe an entity. For example an EMPLOYEE entity may have a Name, TC number, Address, Sex, BirthDate A specific entity will have a value for each of its attributes. For example a specific employee entity may have Name='Cem Yilmaz', TCN='123456789', Address ='Uzun sk No:40 Kadikoy Istanbul Turkey', Sex='M', BirthDate='09-JAN-70 Each attribute has a value set (or data type) associated with it e.g. integer, string, subrange, enumerated type,

Types of Attributes (1) Simple Each entity has a single atomic value for the attribute. For example, TCN or Sex. Composite The attribute may be composed of several components. For example, Address (Apt#, House#, Street, City, State, ZipCode, Country) or Name (FirstName, MiddleName, LastName). Composition may form a hierarchy where some components are themselves composite. Multi-valued An entity may have multiple values for that attribute. For example, Telehone Numbers or PreviousDegrees of a STUDENT. Denoted as {Telephone Number} or {PreviousDegrees}.

Entity Types and Key Attributes Entities with the same basic attributes are grouped or typed into an entity type. For example, the EMPLOYEE entity type or the PROJECT entity type. An attribute of an entity type for which each entity must have a unique value is called a key attribute of the entity type. For example, TCNo of EMPLOYEE. A key attribute may be composite. For example, course no and department code together constitutes a key like CENG 302 (there may be other 302 courses in other departments). An entity type may have more than one key. For example, for a vehicle both of the below numbers are unique Vehicle plate number

Engine number ENTITY SET corresponding to the ENTITY TYPE Movie Movie Name, Year, Language, Director, {Leading Actor}, {Leading Actress} movie1 (Gone with the Wind, 1950, English, Victor Fleming, {Clark Gable},{Vivien Leigh}) movie2 (Hababam Sinifi Merhaba,2004 ,Turkish ,Kartal Tibet, {Halit Akcatepe,M.Ali Erbil, M.Ali Alabora},{Hulya Kocyigit, Nehir Erdogan}) . .

. SUMMARY OF ER-DIAGRAM NOTATION FOR ER SCHEMAS Symbol Meaning ENTITY TYPE WEAK ENTITY TYPE RELATIONSHIP TYPE IDENTIFYING RELATIONSHIP TYPE ATTRIBUTE KEY ATTRIBUTE MULTIVALUED ATTRIBUTE COMPOSITE ATTRIBUTE DERIVED ATTRIBUTE

E1 E1 E2 R R R N (min,max) E2 E TOTAL PARTICIPATION OF E2 IN R CARDINALITY RATIO 1:N FOR E1:E2 IN R

STRUCTURAL CONSTRAINT (min, max) ON PARTICIPATION OF E IN R ER DIAGRAM Entity Types are: EMPLOYEE, DEPARTMENT, PROJECT, DEPENDENT Relationships and Relationship Types (1) A relationship relates two or more distinct entities with a specific meaning. For example, EMPLOYEE Bill Gates works on the Linux Project or EMPLOYEE Cem Yilmaz manages the Entertainment Department. Relationships of the same type are grouped or typed into a relationship type. For example, the WORKS_ON relationship type in which

EMPLOYEEs and PROJECTs participate, or the MANAGES relationship type in which EMPLOYEEs and DEPARTMENTs participate. The degree of a relationship type is the number of participating entity types. Both MANAGES and WORKS_ON are binary relationships. Example relationship instances of the WORKS_FOR relationship between EMPLOYEE and DEPARTMENT EMPLOYEE e1 WORKS_FOR r1 e2

r2 e3 r3 e4 r4 e5 e6 e7 DEPARTMENT

r5 r6 r7 d1 d2 d3 Example relationship instances of the WORKS_ON

relationship between EMPLOYEE and PROJECT r9 e1 r1 e2 r2 e3 r3 e4 r4

e5 e6 e7 r5 r6 r 8 r7 p1 p2

p3 Relationships and Relationship Types (2) More than one relationship type can exist with the same participating entity types. For example, MANAGES and WORKS_FOR are distinct relationships between EMPLOYEE and DEPARTMENT, but with different meanings and different relationship instances. SUMMARY OF ER-DIAGRAM NOTATION FOR ER SCHEMAS Symbol

Meaning ENTITY TYPE WEAK ENTITY TYPE RELATIONSHIP TYPE IDENTIFYING RELATIONSHIP TYPE ATTRIBUTE KEY ATTRIBUTE MULTIVALUED ATTRIBUTE COMPOSITE ATTRIBUTE DERIVED ATTRIBUTE E1 E1 E2 R

R R N (min,max) E2 E TOTAL PARTICIPATION OF E2 IN R CARDINALITY RATIO 1:N FOR E1:E2 IN R STRUCTURAL CONSTRAINT (min, max) ON PARTICIPATION OF E IN R ER DIAGRAM Relationship Types are: WORKS_FOR, MANAGES, WORKS_ON, CONTROLS, SUPERVISION, DEPENDENTS_OF

Weak Entity Types An entity that does not have a key attribute A weak entity must participate in an identifying relationship type with an owner or identifying entity type Entities are identified by the combination of: A partial key of the weak entity type The particular entity they are related to in the identifying entity type Example: Suppose that a DEPENDENT entity is identified by the dependents first name and birhtdate, and the specific EMPLOYEE that the dependent is related to. DEPENDENT is a weak entity type with EMPLOYEE as its identifying entity type via the identifying relationship type DEPENDENT_OF

Weak Entity Type is: DEPENDENT Identifying Relationship is: DEPENDENTS_OF Constraints on Relationships Constraints on Relationship Types ( Also known as ratio constraints ) Maximum Cardinality One-to-one (1:1) One-to-many (1:N) or Many-to-one (N:1) Many-to-many Minimum Cardinality (also called participation constraint or existence dependency constraints) zero (optional participation, not existencedependent) one or more (mandatory, existencedependent)

Many-to-one (N:1) RELATIONSHIP EMPLOYEE e1 WORKS_FOR r1 e2 r2 e3 r3 e4

r4 e5 e6 e7 DEPARTMENT r5 r6 r7 d1

d2 d3 Many-to-many (M:N) RELATIONSHIP r9 e1 r1 e2 r2

e3 r3 e4 r4 e5 e6 e7 r5 r6 r 8 r7

p1 p2 p3 Relationships and Relationship Types (3) We can also have a recursive relationship type. Both participations are same entity type in different roles. For example, SUPERVISION relationships

between EMPLOYEE (in role of supervisor or boss) and (another) EMPLOYEE (in role of subordinate or worker). In following figure, first role participation labeled with 1 and second role participation labeled with 2. In ER diagram, need to display role names to distinguish participations. A RECURSIVE RELATIONSHIP SUPERVISION EMPLOYEE SUPERVISION e1 e2

1 e3 e4 e7 2 r2 2 r3 1 2

e5 e6 r1 2 1 1 1 2 1 r4

r5 2 r6 The Benjamin/Cummings Publishing Company, Inc. 1994, Elmasri/Navathe, Fundamentals of Database Systems, Second Edition Recursive Relationship Type is: SUPERVISION (participation role names are shown) Attributes of Relationship types A relationship type can have attributes; for example, HoursPerWeek of WORKS_ON; its value for each relationship instance describes the number of hours per week that an

EMPLOYEE works on a PROJECT. Attribute of a Relationship Type is: Hours of WORKS_ON Structural Constraints one way to express semantics of relationships Structural constraints on relationships: Cardinality ratio (of a binary relationship): 1:1, 1:N, N:1, or M:N Maximum number of relationship instances that an entity can participate. It is shown by numbers on lines. Participation constraint (on each participating entity type): total (called existence dependency) or partial. Minimum number of relationship instances that an entity can participate. Total participation constraint is

shown by double lining the link. NOTE: These are easy to specify for Binary Relationship Types. Alternative (min, max) notation for relationship structural constraints: Specified on each participation of an entity type E in a relationship type R Specifies that each entity e in E participates in at least min and at most max relationship instances in R Default(no constraint): min=0, max=n Must have min<=max, min>=0, max >=1 Derived from the knowledge of mini-world constraints Examples: A department has exactly one manager and an employee can manage at most one department. Specify (0,1) for participation of EMPLOYEE in MANAGES Specify (1,1) for participation of DEPARTMENT in MANAGES

An employee can work for exactly one department but a department can have any number of employees. Specify (1,1) for participation of EMPLOYEE in WORKS_FOR Specify (1,n) for participation of DEPARTMENT in WORKS_FOR The (min,max) notation relationship constraints (0,1) (1,1) (1,1) (1,N) COMPANY ER Schema Diagram using (min, max) notation

Relationships of Higher Degree Relationship types of degree 2 are called binary Relationship types of degree 3 are called ternary and of degree n are called n-ary In general, an n-ary relationship is not equivalent to n binary relationships Relationships of Higher Degree Assume that you want to model the following structure You keep info about parts, projects and parts For some project, a part is supplied by a certain supplier

Relationships of Higher Degree Higher Degree Relationship vs. Aggregation Some Guidelines First of all, schema design process is an iterative refinement process A concept that is first defined as attribute can be refined into a relationship because it is determined that the atttibute is a reference to another entity type An attribute that exists in several entity types can be promoted to an independent entity

An independent entity that is only related to one other entity type can be defined as attribute of the other entity type UML DESCRIPTION OF COMPANY EXAMPLE Data Modeling Tools A number of popular tools that cover conceptual modeling and mapping into relational schema design. Examples: ERWin, S- Designer (Enterprise Application Suite), ER- Studio, etc.

Some of the Currently Available Automated Database Design Tools COMPANY TOOL FUNCTIONALITY Embarcadero Technologies ER Studio Database Modeling in ER and IDEF1X DB Artisan

Database administration and space and security management Oracle Developer 2000 and Designer 2000 Database modeling, application development Popkin Software System Architect 2001 Data modeling, object modeling, process modeling, structured analysis/design

Platinum Technology Platinum Enterprice Modeling Suite: Erwin, BPWin, Paradigm Plus Data, process, and business component modeling Persistence Inc. Pwertier Mapping from O-O to relational model Rational

Rational Rose Modeling in UML and application generation in C++ and JAVA Rogue Ware RW Metro Mapping from O-O to relational model Resolution Ltd. Xcase Conceptual modeling up to code maintenance

Sybase Enterprise Application Suite Data modeling, business logic modeling Visio Visio Enterprise Data modeling, design and reengineering Visual Basic and Visual C++ ER DIAGRAM FOR A BANK DATABASE The Benjamin/Cummings Publishing Company, Inc. 1994, Elmasri/Navathe, Fundamentals of Database Systems, Second Edition

ER DIAGRAM FOR A BANK DATABASE List the entity types. Tell which ones are non weak and which ones are weak entity types. What constraints do the partial key and identifying relationship of the weak entity types specify in this example List relationships and and tell the participation constraints and existence dependencies for the relationships Suppose that every customer must have at least one account but is restricted to at most two loans at a time, and that a bank branch cannot have more than 1000 loans. How does this show up on the diagram.

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