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MIS 340:Data Modeling 2

Yong Choi

School of Business

CSUB

Entities???• Made up for the class…….ambiguous…

ANG Laboratory has several chemists who work on one or more projects. Chemists also may use certain kinds of equipment on each project. The organization would like to store the chemist’s employee identification number, his/her name, up to three phone numbers, his/her project identification number and the date on which the project started. Every piece of equipment, the chemist uses, has a serial number and a cost.

Entities

Chemist

Project

Equipment

Entities’ Attributes???

ANG Laboratory has several chemists who work on one or more projects. Chemists also may use certain kinds of equipment on each project. The organization would like to store the chemist’s employee identification number, his/her name, up to three phone numbers, his/her project identification number and the date on which the project started. Every piece of equipment, the chemist uses, has a serial number and a cost.

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entities, attributes and identifiers

Project

Proj#Start-Date

Chemist

Phone#Emp#

EquipmentSerial#

cost

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How to find relationships?

• Relationship: – Association between entities– Two entities can have more than one type of relationship

– look for a verb or a verb phrase between entities– A couple of sentences to describe a relationship between two

entities.

More about Relationship• Usually, a relationship can be described by a couple of

sentences between two entities.• Operate in both directions– Relationship between Student and Curriculum

• A student is enrolled in many curriculums.• Each curriculum is being studied by many students.

Relationships ???

ANG Laboratory has several chemists who work on one or more projects. Chemists also may use certain kinds of equipment on each project. The organization would like to store the chemist’s employee identification number, his/her name, up to three phone numbers, his/her project identification number and the date on which the project started. Every piece of equipment, the chemist uses, has a serial number and a cost.

Entities/Relationships& their Attributes

Chemist

Phone#

Project

Proj#Start-Date

Equipment

Works-On

Uses

Date-Assigned

Emp#

Serial#

cost

Assign-Date

Steps for creating an ERD• Identify the entities

– Beginner: look for nouns• Identify the attributes

– Beginner: look for entity descriptions• Identify the relationships

– Beginner: look for a verb or a verb phrase between entities

Degree of Relationship

• Degree of a Relationship describes the number of entity participation – Unary (Recursive) Relationship: One instance

related to another of the same entity type– Binary Relationship: Instances of two different

entities related to each other– Ternary Relationship: Instances of three

different types related to each other

Degree of Relationship …

Type of Relationships (Cardinality)• One – to – One (1:1)

– Each instance in the relationship will have exactly one related member on the other side

• One – to – Many (1:M)– A instance on one side of the relationship can have many

related members on the other side, but a member on the other side will have a maximum of one related instance

• Many – to – Many (M:N)– Instances on both sides of the relationship can have many

related instances on the other side

1:1 relationship in Set notation

DEPARTMT EMPLOYEE

Example of 1:1 relationship

• A one-to-one (1:1) relationship is when at most one instance of an entity A is associated with one instance of entity B.– Each employee is assigned to one workstation. – Not all workstations are assigned to employees.

1:M relationship in Set notation

DEPARTMT EMPLOYEE

Example of 1:M relationship

• A one-to-many (1:N) relationships is when for one instance of entity A, there are zero, one, or many instances of entity B, but for one instance of entity B, there is only one instance of entity A.– A department is responsible for many projects. – Each project is the responsibility of one department.

M:N relationship in Set notation

WAREHOUSE PRODUCT

Example of M:N relationship

• A many-to-many (M:N) relationship, sometimes called non-specific, is when for one instance of entity A, there are zero, one, or many instances of entity B and for one instance of entity B there are zero, one, or many instances of entity A. – Employees are assigned to many projects. – Every project has assigned at least one employee.

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Type of Relationship (Cardinality)The organization would like to store the date the chemist was assigned to the project and the date an equipment item was assigned to a particular chemist working on a particular project. A chemist must be assigned at least to one (or more) project and one (or more) equipment. Projects and equipments must be managed by only one chemist. A given project need not be assigned an equipment.

Complete ER Diagram

Chemist

Phone#

Project

Proj#Start-Date

Equipment

Works-On

Uses

Date-Assigned

N1

N1

Emp#

Serial#

cost

Assign-Date

Steps for creating an ERD• Find out candidate entities• Identify the entities • Identify the attributes • Identify the relationships

– Beginner: look for participation related words and phrases such as zero, none, a, one, several, many…..

– Optional relationship: look for auxiliary verbs such as may, might, can and based upon own judgment..)

• Finalize business rules

Original IE Notations(minor differences with the textbook – pp.223)

• 1-to-1 relationship

• 1-to-M relationship

• M-to-N relationship

• weak entity relationship

• optional relationship

• recursive relationship

Employee

Type Minimum Instances

Maximum Instances

Graphic Notation

Exactly one

Zero or one

One or many

Zero or many

1

1

0

0

1

1

Many (>1)

Many (>1)

1:1 relationship

A person must have one and only one DNA pattern and that pattern must be applied to one and only one person.

1:1 with optional relationship (OR)on one side

A person might not or might be a programmer, but a programmer must be a person.

1:M relationship

Each department hires many employees, and each employee is hired by one department.

EMPLOYEEDEPARTMTHIRES

IS_HIRED_BY

1:M with OR on many side A person might be a member or might not, but could be

found multiple times (if the member entity represents membership in multiple clubs, for instance). A member must have only a single person.

1:M with OR on both side A person might have no phone, one phone or lots of

phones, and that a phone might be un-owned or can only be owned by a person.

M:N relationshipEach student takes many classes, and a class must be taken by many students.

** Many-to-many relationships cannot be used in the data model because they cannot be represented by the relational model (see the next slide for the reason) **

STUDENTCLASSTAKE

IS_TAKEN_BY

Example of M:N

Many-to-many relationships is a second sign of complex data.

When x relates to many y's and y relates to many x's, it is a many-to-many relationship.

In our example schema, a color swatch can relate to many types of sweaters and a type of sweater can have many color swatches. 

Example M:N Relationship

3 to 330 to 30

300 to 3003000 to 3000

30,000 to 30,000300, 000 to 300, 000

Table to represent Entity

Transformation of M:N1. When transform to relational model, many

redundancies can be generated.– The relational operations become very complex and are likely

to cause system efficiency errors and output errors. – Break the M:N down into 1:N and N:1 relationships using

bridge entity (weak entity).

CLASS STUDENTENROLL

Weak Entity relationship

• A weak entity is an entity that cannot be uniquely identified and existed by itself alone.

• Thus, a weak entity is an entity that exists only if it is related to a set of uniquely determined entities (owners of the weak entity). – More examples on the textbook

• Each employee might have none or multiple dependents. However, dependents must belong to at least one employee.

EMP DEP

weak entity notation

Converting M:N Relationship to Two 1:M Relationships

Bridge Entity

Bridge Entity

• ENROLL entity becomes a weak entity of both STUDENT entity and CLASS entity

• MUST have a composite (unique) identifier STU_NUM (from STUDENT entity) and CLASS_CODE

(from CLASS entity)

M:N with optionality on both side

• A person might or might not work for an employer, but could certainly moonlight for multiple companies. An employer might have no employees, but could have any number of them.

After broken down, optional relationship notation on both side of associative entity

Recursive relationship 1

• A recursive relationship is an entity is associated with itself.

• Each employee is supervised directly by at most one supervisor (manager). Each supervisor (manager) can manage many employees.

Employee

manages

is managed by

Recursive relationship 2

• Each student is taught by a STA (student teaching assistant). Each STA can teach several students.

Student

teaches

is taught by

Data Modeling Errors

• In general, there are two classes of E-R modeling errors that lead to normalization problems: – Incomplete data model error– Miss-modeled problem domain error

• Read next two slides…

In Complete Data Model

• Occur in situations where the systems analysts is tasked to build a computer-based information system that is limited in scope. A key objective for successful information system project management is the definition of a limited, yet adequate project scope--a scope that enables the production of system deliverables within a reasonable time period. Limiting a project's scope often results in information systems that are based on limited data models. Limited information systems are fairly common throughout the IS world where dissimilar technologies prevent data sharing and work against the concept of a shared, enterprise-wide database.

Miss-modeled problem domain error

• The miss-modeled problem domain error is actually a class of errors including those that arise whenever systems analysts lack a complete understanding of the problem domain. These include errors such as depicting an attribute as single-valued when, in fact, the attribute is multi-valued, or depicting a single entity which includes attributes that should be assigned to two separate entities, or miss-modeling the connectivity or degree of a relationship.