Download Entities

Data Modeling 2
Yong Choi
School of Business
CSUB
1
Steps for creating an ERD
• Identify the entities
– Look for singular nouns (in real world situation, avoid
noun w/o attributes)
– Also avoid proper nouns
• Identify the attributes
– Look for entity characteristics or properties
• Identify the relationships
– Look for verb (between entities)
Entities???
• Made up for the class…….ambiguous…
•Using original Chen’s notation
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
Project
Chemist
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.
entities, attributes and identifiers
Emp#
Phone#
Chemist
Equipment
6
Proj#
Start-Date
Project
Serial#
cost
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
Start-Date
Proj#
Project
Works-On
Emp#
Phone#
Date-Assigned
Chemist
Equipment
Uses
Assign-Date
Serial#
cost
More about Relationship
• Description of each relationship should be bidirectional.
• Operate in both directions
– Relationship between Student and Curriculum
• A student is enrolled in many curriculums (student to
Curr.).
• Each curriculum is being studied by many students (Curr to
student).
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
Relationships
• Entities can be associated with one
another in relationships.
• Relationship degree defines the number
of entity classes participating in the
relationship:
– Unary relationship.
– binary relationship.
– ternary relationship.
Degree of Relationship …
Unary (recursive) Relationship
• It is possible for an entity to have a relationship to
itself—this is called a recursive relationship.
Is supervised by
supervises
Binary Relationship
Ternary Relationship
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
Optionality
• The Optionality is a property of an attribute which
specify if a value is mandatory or optional.
• To identify optional relationship, look for
auxiliary verb such as can or may
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.
18
Complete ER Diagram
Start-Date
Proj#
1
Emp#
Works-On
N
Project
Phone#
Date-Assigned
Chemist
1
Uses
N
Equipment
Assign-Date
Serial#
cost
Steps for developing an ERD 2
• Identify the entities
• Identify the attributes
• Identify the relationships
– Beginner: look for relationship-type 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
Data Model Notation
• 1-to-1 relationship
• 1-to-M relationship
• M-to-N relationship
Data Model Notation
Data Model Notation
• weak entity relationship (see the next slide)
• optional relationship
• recursive relationship
Employee
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
1:1 relationship
A person must have one and only one DNA pattern
and eac 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.
HIRES
DEPARTMT
IS_HIRED_BY
EMPLOYEE
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 relationship
Each student takes many classes, and a class must be
taken by many students.
IS_TAKEN_BY
CLASS
STUDENT
TAKE
** Many-to-many relationships cannot allowed in the data
model because they cannot be represented by the
relational model (see the next slide for the reason) **
Example M:N Relationship
Table to represent Entity
3 to 3
30 to 30
300 to 300
3000 to 3000
30,000 to 30,000
300, 000 to 300, 000
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.
Transformation of M:N
1. When transform to relational model, many
redundancies can be generated.
–
–
CLASS
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).
ENROLL
STUDENT
Converting M:N Relationship to Two 1:M Relationships
Bridge Entity
Bridge (Associative - textbook) 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)
• Another MUST know M:N example on the
textbook page 63 and 64
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
• Each student is taught by a STA (student teaching
assistant). Each STA can teach several students.
• A recursive relationship is an entity is associated with
itself.
teaches
Student
is taught by