1

CS 338: Computer Applications in Business: Databases (Fall 2014)

1992-2014 by Addison Wesley & Pearson Education, Inc., McGraw Hill, Cengage Learning Slides adapted and modified from Fundamentals of Database Systems (5/6) (Elmasri et al.), Database System Concepts (5/6) (Silberschatz et al.), Database Systems (Coronel et al.), Database Systems (4/5) (Connolly et al. ), Database Systems: Complete Book (Garcia-Molina et al.)

CS 338: Computer Applications in Business: Databases

The Entity-Relationship (ER) Model

1992-2014 by Addison Wesley & Pearson Education, Inc., McGraw Hill, Cengage Learning Slides adapted and modified from Fundamentals of Database Systems (5/6) (Elmasri et al.), Database System Concepts (5/6) (Silberschatz et al.), Database Systems (Coronel et al.), Database Systems (4/5) (Connolly et al. ), Database Systems: Complete Book (Garcia-Molina et al.) Rice University Data Center

Fall 2014

Chapter 7

Phases of Database Design

Generally, the term database application refers to a particular database and the associated programs that implement the database queries and updates Traditionally, design of application programs is typically covered in

software engineering courses

3

Conceptual Modeling is a very important phase in designing a successful database application

Lets consider the process whereby a new database, such as our company database is created. Where do we begin? We begin with a design phase in which we address and answer questions about what information will be stored, how information elements will be related to one another, what constraints such as keys or referential integrity may be assumed, among others.

consider this

ideas High-Level Design

Relational Database Schema

Relational DBMS

2

CS 338: Computer Applications in Business: Databases (Fall 2014)

1992-2014 by Addison Wesley & Pearson Education, Inc., McGraw Hill, Cengage Learning Slides adapted and modified from Fundamentals of Database Systems (5/6) (Elmasri et al.), Database System Concepts (5/6) (Silberschatz et al.), Database Systems (Coronel et al.), Database Systems (4/5) (Connolly et al. ), Database Systems: Complete Book (Garcia-Molina et al.)

Phases of Database Design

ER diagrams is diagrammatic notation associated with the ER model

4

Entity-Relationship (ER) model is a popular high-level conceptual data model used for the conceptual design of database applications

1. Requirements Collection and Analysis Database designers interview prospective database users to understand and document data

requirements Result: set of user requirements (should be as detailed as possible) In parallel, it is useful to specify the known functional requirements of the application

2. Conceptual Database Design Detailed description of the data requirements (components and constraints) Result: conceptual schema in a high-level, conceptual data model

3. Logical Database Design (or Data Model Mapping) Transform the conceptual model into implementation data model Most commercial DBMSs use an implementation data model (e.g. relational or object-relational) Result: Database schema in implementation data model of DBMS

4. Physical Database Design Internal storage structures, file organizations, indexes, access paths, and physical design

parameters for the database files

Steps in Database Design

Sample Database Application: COMPANY

5

COMPANY database keeps track of employees, departments and projects 1. Company is organized into departments

Each department has a unique name, a unique number, and a particular employee who manages the department

Keep track of the start date when the employee began managing the department A department may have several locations

2. A department controls a number of projects Each project has a unique name, a unique number, and a single location

3. Employee: require each employees Social Security Number, name, address, salary, gender, and birth date An employee is assigned to one department, but may work on several projects (which

are not necessarily controlled by the same department) Keep track of the current number of hours per week that an employee works on each

project Also keep track of the direct supervisor of each employee (who is another employee)

4. Keep track of the dependents of each employee for insurance purposes Keep track of the dependents first name, sex, birth date, and relationship to the

employee

Requirements gathered for COMPANY

3

CS 338: Computer Applications in Business: Databases (Fall 2014)

1992-2014 by Addison Wesley & Pearson Education, Inc., McGraw Hill, Cengage Learning Slides adapted and modified from Fundamentals of Database Systems (5/6) (Elmasri et al.), Database System Concepts (5/6) (Silberschatz et al.), Database Systems (Coronel et al.), Database Systems (4/5) (Connolly et al. ), Database Systems: Complete Book (Garcia-Molina et al.)

ER Model Overview Three Principal Element Types

6

An entity is an abstract object of some sort Example: Person, Car, House, Employee A collection of similar entities forms an entity set

Entities and Entity Sets

Let us consider the design of our running company-database example. Each department is an entity, and the set of all departments constitutes an entity set. Likewise, the employees are entities, and the set of employees is an entity set. A project is another entity and the set of projects is a third entity set.

Example 1

Each entity has attributes the particular properties that describe it Example: an employee entity may be described by the employees name, age, address,

and position A particular entity will have a value for each of its attributes the attribute values that

describe each entity become a major part of the data stored in the database

Attributes

Relationships are connections among two or more entity sets Example: If Employee(s) and Department(s) are two entity sets, we could have a

relationship Works-for that connects employee and department

Relationships

Entities and Attributes Example

7

4

CS 338: Computer Applications in Business: Databases (Fall 2014)

1992-2014 by Addison Wesley & Pearson Education, Inc., McGraw Hill, Cengage Learning Slides adapted and modified from Fundamentals of Database Systems (5/6) (Elmasri et al.), Database System Concepts (5/6) (Silberschatz et al.), Database Systems (Coronel et al.), Database Systems (4/5) (Connolly et al. ), Database Systems: Complete Book (Garcia-Molina et al.)

Categories of Attributes

Simple (atomic) attributes vs. composite attributes

Single-valued attributes vs. multivalued attributes

Stored attributes vs. derived attributes

Key (or unique) attributes Attribute values constrained to be distinct for individual entities in

an entity set e.g. SSN

8

Categories of Attributes Composite vs. Simple (Atomic) Attributes

9

Can be divided into smaller subparts, which represent more basic attributes with independent meanings

Composite attributes can form a hierarchy: the value of a composite attribute is the concatenation of the values of its component simple attributes

If the composite attribute is referenced only as a whole, there is no need to subdivide it into component attributes

Composite Attributes

Attributes that are not divisible

Simple (Atomic) Attributes

Nathan WilsonSimple/composite:Deals with attribute type

Single/multivalued:Deals with attribute values

Stored/derived:Deals with how attribute value is obtained

5

CS 338: Computer Applications in Business: Databases (Fall 2014)

1992-2014 by Addison Wesley & Pearson Education, Inc., McGraw Hill, Cengage Learning Slides adapted and modified from Fundamentals of Database Systems (5/6) (Elmasri et al.), Database System Concepts (5/6) (Silberschatz et al.), Database Systems (Coronel et al.), Database Systems (4/5) (Connolly et al. ), Database Systems: Complete Book (Garcia-Molina et al.)

Categories of Attributes Single-valued vs. Multivalued Attributes

10

Most attributes have a single value for a particular entity Example: Age is a single-valued attribute of person

Single Attributes

In some cases, an attribute can have a set of values for the same entity Example: a Colors attribute for a car Example: Different people can have numbers of values for the College_degrees attribute Multivalued attribute can have lower and upper bounds to constrain the number of values

allowed for each individual entity Example: the Colors attribute of a car may be restricted to have between one and three

values (assuming that a car can have three colors at most)

Multivalued Attributes

Categories of Attributes Stored vs. Derived Attributes

11

In some cases, two (or more) attribute values are related

Consider Age and Birth_date attributes of a person The value of Age can be determined from the current (todays) date

and the value of that persons Birth_date

Age attribute is hence called a derived attribute and is said to be derivable from the Birth_date attribute, which is called a stored attribute

Some attribute values can be derived from related entities (Example: NumberOfEmployees)

Stored vs. Derived Attributes

6

CS 338: Computer Applications in Business: Databases (Fall 2014)

1992-2014 by Addison Wesley & Pearson Education, Inc., McGraw Hill, Cengage Learning Slides adapted and modified from Fundamentals of Database Systems (5/6) (Elmasri et al.), Database System Concepts (5/6) (Silberschatz et al.), Database Systems (Coronel et al.), Database Systems (4/5) (Connolly et al. ), Database Systems: Complete Book (Garcia-Molina et al.)

NULL Values

12

In some cases, a particular entity may not have an applicable value for an attribute

Example: ApartmentNumber attribute of an address applies only to

addresses that are in Apartment Buildings and NOT to other types of residences (i.e. house)

NULL Values

Entity Sets

13

Entity type (or entity set) Collection (or set) of similar entities that have the same attributes An entity type describes the schema or intension of a set of entities

that share the same structure The collection of entities of a particular entity type is grouped into an

entity set (also known as the extension of the entity type)

ER model defines entity sets, not individual entities But entity sets are described in terms of their attributes

7

CS 338: Computer Applications in Business: Databases (Fall 2014)

1992-2014 by Addison Wesley & Pearson Education, Inc., McGraw Hill, Cengage Learning Slides adapted and modified from Fundamentals of Database Systems (5/6) (Elmasri et al.), Database System Concepts (5/6) (Silberschatz et al.), Database Systems (Coronel et al.), Database Systems (4/5) (Connolly et al. ), Database Systems: Complete Book (Garcia-Molina et al.)

Displaying an Entity Type in ER Models

In ER diagrams, an entity type is displayed as a rectangular box enclosing entity name

Attributes are displayed as ovals Each attribute is

connected to its entity type by a straight line

Composite attributes are attached to their component attributes by straight lines

Multivalued attributes are displayed in double ovals

14

Sample Database Application: COMPANY Revisited

COMPANY database keeps track of employees, departments and projects 1. Company is organized into DEPARTMENTS

Each department has a unique name, a unique number, and a particular employee who manages the department.

Keep track of the start date when the employee began managing the department. A department may have several locations

2. A department controls a number of PROJECTS Each project has a unique name, a unique number, and a single location

3. EMPLOYEE: require each employees Social Security Number, name, address, salary, gender, and birth date An employee is assigned to one department, but may work on several projects (which

are not necessarily controlled by the same department) Keep track of the current number of hours per week that an employee works on each

project Also keep track of the direct supervisor of each employee (who is another employee)

4. Keep track of the DEPENDENTS of each employee for insurance purposes Keep track of the dependents first name, sex, birth date, and relationship to the

employee

Requirements gathered for COMPANY

15

8

CS 338: Computer Applications in Business: Databases (Fall 2014)

1992-2014 by Addison Wesley & Pearson Education, Inc., McGraw Hill, Cengage Learning Slides adapted and modified from Fundamentals of Database Systems (5/6) (Elmasri et al.), Database System Concepts (5/6) (Silberschatz et al.), Database Systems (Coronel et al.), Database Systems (4/5) (Connolly et al. ), Database Systems: Complete Book (Garcia-Molina et al.)

Sample Database Application: COMPANY Revisited

Based on the requirements, we can identify four initial entity types in the COMPANY database:

DEPARTMENT

PROJECT

EMPLOYEE

DEPENDENT

16

Initial ER Diagram for COMPANY

Four entity types

Most attributes are simple, single-valued, and stored Works_on and Locations are multivalued Employees name is composite

Employee has one key, dependent has none, department and project have two keys

17

9

CS 338: Computer Applications in Business: Databases (Fall 2014)

1992-2014 by Addison Wesley & Pearson Education, Inc., McGraw Hill, Cengage Learning Slides adapted and modified from Fundamentals of Database Systems (5/6) (Elmasri et al.), Database System Concepts (5/6) (Silberschatz et al.), Database Systems (Coronel et al.), Database Systems (4/5) (Connolly et al. ), Database Systems: Complete Book (Garcia-Molina et al.)

Weak Entity Types

Entity types that do not have key attributes of their own Identified by their relationship to specific entities from another entity type Weak entity types are represented by surrounding their boxes and diamonds with

double lines

Identifying relationship Relates a weak entity type to the identifying entity, which has the rest of the key

18

Dependent is meaningless in COMPANY DB independently of Employee

Identified by relationship to Employee

Dependent_name distinguishes one dependent from other dependents for the same employee: partial key

Sample Database Application: COMPANY Revisited

Initial design is typically not complete We have not represented that an employee can work on several

projects We have not represented the number of hours per week an

employee works on

Some aspects in the requirements will be represented as relationships Example: attribute Manager of DEPARTMENT refers to an

employee who manages the department Example: Controlling_department of PROJECT refers to the

department that controls the project

In the ER model, these references should not be represented as attributes but as relationships

19

10

CS 338: Computer Applications in Business: Databases (Fall 2014)

1992-2014 by Addison Wesley & Pearson Education, Inc., McGraw Hill, Cengage Learning Slides adapted and modified from Fundamentals of Database Systems (5/6) (Elmasri et al.), Database System Concepts (5/6) (Silberschatz et al.), Database Systems (Coronel et al.), Database Systems (4/5) (Connolly et al. ), Database Systems: Complete Book (Garcia-Molina et al.)

Relationships in General

Relationship An association (or interaction) between entities Indicator: an attribute of one entity refers to another entity Represent such references as relationships not attributes

Examples: EMPLOYEE John Smith Works on the ProductX PROJECT

20

Relationships

Relationship An association (or interaction) between entities Indicator: an attribute of one entity refers to another entity Represent such references as relationships not attributes

Relationship type R among n entity types E1, E2, ..., En Defines a set of associations among entities from these entity types

Relationship instance ri Each ri associates n individual entities (e1, e2, ..., en) Each entity ej in ri is a member of entity set Ej Relationships uniquely identified by keys of participating entities

Degree of a relationship type Number of participating entity types e.g., binary, ternary

21

11

CS 338: Computer Applications in Business: Databases (Fall 2014)

1992-2014 by Addison Wesley & Pearson Education, Inc., McGraw Hill, Cengage Learning Slides adapted and modified from Fundamentals of Database Systems (5/6) (Elmasri et al.), Database System Concepts (5/6) (Silberschatz et al.), Database Systems (Coronel et al.), Database Systems (4/5) (Connolly et al. ), Database Systems: Complete Book (Garcia-Molina et al.)

Degree of Relationship Examples

In ER diagrams, we represent the relationship type as follows

Diamond-shaped box is used to display a relationship type

Connected to the participating entity types via straight lines

22

Relationships with Repeated Entity Sets

23

Some relationships involve multiple entities from the same entity set e.g., spouse (two persons), games (two

teams) e.g., recursive relationships, such as

supervises (two employees)

Recursive relationships A relationship type with the same

participating entity type but in different roles

Role name Signifies role that participating entity

plays in relationship instance Required when entity type participates

multiple times in a relationship

12

CS 338: Computer Applications in Business: Databases (Fall 2014)

1992-2014 by Addison Wesley & Pearson Education, Inc., McGraw Hill, Cengage Learning Slides adapted and modified from Fundamentals of Database Systems (5/6) (Elmasri et al.), Database System Concepts (5/6) (Silberschatz et al.), Database Systems (Coronel et al.), Database Systems (4/5) (Connolly et al. ), Database Systems: Complete Book (Garcia-Molina et al.)

Relationship Constraints

Cardinality ratio Specifies maximum number of relationship instances in which each entity

can participate Types 1:1, 1:N, or M:N

Participation constraint

Specifies whether existence of entity depends on its being related to another entity

Types: total and partial Thus, minimum number of relationship instances in which entities can

participate: At least 1 for total participation At least 0 for partial

Diagrammatically, use a double line from relationship type to entity type

24

Relationship Constraints

Partial Participation If there is no participation constraint, it is possible that an entity will not

participate in a relationship set

Total Participation If there is a participation constraint, an entity must participate at least once

Presented by double line from entity set to relationship set

Alternative: Structural constraint Specify any min and max participation Generalization of cardinality ratio and participation constraint

E1 E2 R

E1 E2 R

25

13

CS 338: Computer Applications in Business: Databases (Fall 2014)

1992-2014 by Addison Wesley & Pearson Education, Inc., McGraw Hill, Cengage Learning Slides adapted and modified from Fundamentals of Database Systems (5/6) (Elmasri et al.), Database System Concepts (5/6) (Silberschatz et al.), Database Systems (Coronel et al.), Database Systems (4/5) (Connolly et al. ), Database Systems: Complete Book (Garcia-Molina et al.)

Relationship Attributes

Relationship types can also have attributes Property that depends on both/all participating entities Example: Percentage of control that department has on a project

Attributes of 1:1 or 1:N relationship types can be migrated to one of the participating entity types For a 1:N relationship type, relationship attribute can

be migrated only to entity type on N-side of relationship Attributes on M:N relationship types must be specified

as relationship attributes

26

Percent CONTROLS

Summary of Notation for ER Diagrams

Continued next page

14

CS 338: Computer Applications in Business: Databases (Fall 2014)

1992-2014 by Addison Wesley & Pearson Education, Inc., McGraw Hill, Cengage Learning Slides adapted and modified from Fundamentals of Database Systems (5/6) (Elmasri et al.), Database System Concepts (5/6) (Silberschatz et al.), Database Systems (Coronel et al.), Database Systems (4/5) (Connolly et al. ), Database Systems: Complete Book (Garcia-Molina et al.)

Summary of Notation for ER Diagrams

Appropriate ER Model Design

Choose names that convey meanings attached to different constructs in schema

Nouns give rise to entity type names

Verbs indicate names of relationship types

Entities that must participate in a relationship with another entity type and with cardinality constraint of 1 might be better modeled as weak entity