ERmodel (2).pdf

UCSD CSE132B Slide 1
CSE 132B
CSE 132B
Database Systems Principles
Database Systems Principles
Alin Deutsch
Database Design with the Entity-Relationship (ER) Model
Some slides are based or modified from originals by
Elmasri and Navathe,
Fundamentals of Database Systems, 4th Edition
© 2004 Pearson Education, Inc.
and
Database System Concepts, McGraw Hill 5th Edition
© 2005 Silberschatz, Korth and Sudarshan

Database Modeling
Database Modeling
A database can be modeled as:
a collection of entities,
relationship among entities.
An entity is an object that exists and is distinguishable from other
objects.
Example: specific person, company, event, plant
Entities have attributes
Example: people have names and addresses
An entity set is a set of entities of the same type that share the same
properties.
Example: set of all persons, companies, trees, holidays

Entity Sets
Entity Sets customer
customer and
and loan
loan
customer_id customer_ customer_ customer_ loan_ amount
name street city number

Attributes
Attributes
An entity is represented by a set of attributes, that is descriptive
properties possessed by all members of an entity set.
Domain – the set of permitted values for each attribute
Attribute types:
Simple and composite attributes.
Single-valued and multi-valued attributes
Example: multivalued attribute: phone_numbers
Derived attributes
Can be computed from other attributes
Example: age, given date_of_birth
Example:
customer = (customer_id, customer_name,
customer_street, customer_city )
loan = (loan_number, amount )

Composite Attributes
Composite Attributes

Relationship Sets
Relationship Sets
A relationship is an association among several entities
Example:
Hayes bottower L-15
customer entity relationship set loan entity
A relationship set is a mathematical relation among n ≥ 2 entities, each
taken from entity sets E1, E2, …, En (called the participating entity sets)
{(e1, e2, … en) | e1 ∈ E1, e2 ∈ E2, …, en ∈ En}
where (e1, e2, …, en) is a relationship
Example:
(Hayes, L-15) ∈ borrower

Relationship Set
Relationship Set borrower
borrower

Relationship Sets
Relationship Sets (Cont.)
(Cont.)
An attribute can also be property of a relationship set.
For instance, the depositor relationship set between entity sets customer
and account may have the attribute access-date

Degree of a Relationship Set
Degree of a Relationship Set
Refers to number of entity sets that participate in a relationship set.
Relationship sets that involve two entity sets are binary (or degree two).
Generally, most relationship sets in a database system are binary.
Relationship sets may involve more than two entity sets.
Relationships between more than two entity sets are rare. Most
relationships are binary.
Example: Suppose employees of a bank may have jobs
(responsibilities) at multiple branches, with different jobs at
different branches. Then there is a ternary relationship set
between entity sets employee, job, and branch

Mapping Cardinality Constraints
Mapping Cardinality Constraints
Express the number of entities to which another entity can be
associated via a relationship set.
Most useful in describing binary relationship sets.
For a binary relationship set the mapping cardinality must be one of
the following types:
One to one
One to many
Many to one
Many to many

Mapping Cardinalities
One to one One to many
Note: Some elements in A and B may not be mapped to any
elements in the other set

Many to one Many to many
Note: Some elements in A and B may not be mapped to any
elements in the other set

Keys
Keys
A super key of an entity set is a set of one or more attributes
whose values uniquely determine each entity.
A candidate key of an entity set is a minimal super key
Customer_id is candidate key of customer
account_number is candidate key of account
Although several candidate keys may exist, one of the candidate
keys is selected to be the primary key.
Obs. Entities sometimes are considered to have only keys, without
distinction about being primary or not!

Keys for Relationship Sets
Keys for Relationship Sets
The combination of primary keys of the participating entity sets forms a
super key of a relationship set.
(customer_id, account_number) is the super key of depositor
NOTE: this means a pair of entity sets can have at most one
relationship in a particular relationship set.
Example: if we wish to track all access_dates to each account by
each customer, we cannot assume a relationship for each
access. We can use a multivalued attribute though
Must consider the mapping cardinality of the relationship set when
deciding the what are the candidate keys
Need to consider semantics of relationship set in selecting the primary
key in case of more than one candidate key

E
E-
-R Diagrams
R Diagrams
Rectangles represent entity sets.
Diamonds represent relationship sets.
Lines link attributes to entity sets and entity sets to relationship sets.
Ellipses represent attributes
Double ellipses represent multivalued attributes.
Dashed ellipses denote derived attributes.
Underline indicates primary key attributes (will study later)

E
E-
-R Diagram With Composite, Multivalued, and
R Diagram With Composite, Multivalued, and
Derived Attributes
Derived Attributes

Relationship Sets with Attributes
Relationship Sets with Attributes

Roles
Roles
Entity sets of a relationship need not be distinct
The labels “manager” and “worker” are called roles; they specify how
employee entities interact via the works_for relationship set.
Roles are indicated in E-R diagrams by labeling the lines that connect
diamonds to rectangles.
Role labels are optional, and are used to clarify semantics of the
relationship

Cardinality Constraints
Cardinality Constraints
We express cardinality constraints by drawing either a directed line (→),
signifying “one,” or an undirected line (—), signifying “many,” between
the relationship set and the entity set.
One-to-one relationship:
A customer is associated with at most one loan via the relationship
borrower
A loan is associated with at most one customer via borrower

One
One-
-To
To-
-Many Relationship
Many Relationship
In the one-to-many relationship a loan is associated with at most one
customer via borrower, a customer is associated with several (including
0) loans via borrower

Many
Many-
-To
To-
-One Relationships
One Relationships
In a many-to-one relationship a loan is associated with several
(including 0) customers via borrower, a customer is associated with at
most one loan via borrower

Many
Many-
-To
To-
-Many Relationship
Many Relationship
A customer is associated with several (possibly 0) loans via
borrower
A loan is associated with several (possibly 0) customers via
borrower

Participation of an Entity Set in a Relationship Set
Participation of an Entity Set in a Relationship Set
Total participation (indicated by double line): every entity in the entity set
participates in at least one relationship in the relationship set
E.g. participation of loan in borrower is total
every loan must have a customer associated to it via borrower
Partial participation: some entities may not participate in any relationship in
the relationship set
Example: participation of customer in borrower is partial

Alternative Notation for Cardinality Limits
Alternative Notation for Cardinality Limits
Cardinality limits can also express participation constraints

E
E-
-R
R Diagram with a Ternary Relationship
Diagram with a Ternary Relationship

Cardinality Constraints on Ternary Relationship
Cardinality Constraints on Ternary Relationship
We allow at most one arrow out of a ternary (or greater degree) relationship
to indicate a cardinality constraint
E.g. an arrow from works_on to job indicates each employee works on at
most one job at any branch.
If there is more than one arrow, there are two ways of defining the meaning.
E.g a ternary relationship R between A, B and C with arrows to B and C
could mean
1. each A entity is associated with a unique entity from B and C or
2. each pair of entities from (A, B) is associated with a unique C entity,
and each pair (A, C) is associated with a unique B
Each alternative has been used in different formalisms
To avoid confusion we outlaw more than one arrow

Design Issues
Design Issues
Use of entity sets vs. attributes
Choice mainly depends on the structure of the enterprise being
modeled, and on the semantics associated with the attribute in
question.
Use of entity sets vs. relationship sets
Possible guideline is to designate a relationship set to describe an
action that occurs between entities
Binary versus n-ary relationship sets
Although it is possible to replace any nonbinary (n-ary, for n 2)
relationship set by a number of distinct binary relationship sets, a
n-ary relationship set shows more clearly that several entities
participate in a single relationship.
Placement of relationship attributes

Weak Entity Sets
Weak Entity Sets
An entity set that does not have a primary key is referred to as a weak
entity set.
The existence of a weak entity set depends on the existence of a
identifying entity set
it must relate to the identifying entity set via a total, one-to-many
relationship set from the identifying to the weak entity set
Identifying relationship depicted using a double diamond
The discriminator (or partial key) of a weak entity set is the set of
attributes that distinguishes among all the entities of a weak entity set.
The primary key of a weak entity set is formed by the primary key of the
strong entity set on which the weak entity set is existence dependent,
plus the weak entity set’s discriminator.

Weak Entity Sets (Cont.)
We depict a weak entity set by double rectangles.
We underline the discriminator of a weak entity set with a dashed
line.
payment_number – discriminator of the payment entity set
Primary key for payment – (loan_number, payment_number)

Weak Entity Sets
(Cont.)
Note: the primary key of the strong entity set is not explicitly stored
with the weak entity set, since it is implicit in the identifying
relationship.
If loan_number were explicitly stored, payment could be made a
strong entity, but then the relationship between payment and loan
would be duplicated by an implicit relationship defined by the
attribute loan_number common to payment and loan

More Weak Entity Set Examples
More Weak Entity Set Examples
In a university, a course is a strong entity and a course_offering can
be modeled as a weak entity
The discriminator of course_offering would be semester (including
year) and section_number (if there is more than one section)
If we model course_offering as a strong entity we would model
course_number as an attribute.
Then the relationship with course would be implicit in the
course_number attribute

Extended E
Extended E-
-R Features: Specialization
R Features: Specialization
Top-down design process; we designate subgroupings within an entity set
that are distinctive from other entities in the set.
These subgroupings become lower-level entity sets that have attributes or
participate in relationships that do not apply to the higher-level entity set.
Depicted by a triangle component labeled ISA (E.g. customer “is a”
person).
Attribute inheritance – a lower-level entity set inherits all the attributes
and relationship participation of the higher-level entity set to which it is
linked.

Specialization Example
Specialization Example

Extended ER Features: Generalization
Extended ER Features: Generalization
A bottom-up design process – combine a number of entity sets
that share the same features into a higher-level entity set.
Specialization and generalization are simple inversions of each
other; they are represented in an E-R diagram in the same way.
The terms specialization and generalization are used
interchangeably.

Specialization and Generalization
Specialization and Generalization (Cont.)
(Cont.)
Can have multiple specializations of an entity set based on different
features.
E.g. permanent_employee vs. temporary_employee, in addition to
officer vs. secretary vs. teller
Each particular employee would be
a member of one of permanent_employee or temporary_employee,
and also a member of one of officer, secretary, or teller
The ISA relationship also referred to as superclass - subclass
relationship

Design Constraints on a
Design Constraints on a
Specialization/Generalization
Constraint on which entities can be members of a given lower-level
entity set.
condition-defined
Example: all customers over 65 years are members of senior-
citizen entity set; senior-citizen ISA person.
user-defined
Constraint on whether or not entities may belong to more than one
lower-level entity set within a single generalization.
Disjoint
an entity can belong to only one lower-level entity set
Noted in E-R diagram by writing disjoint next to the ISA
triangle
Overlapping
an entity can belong to more than one lower-level entity set

Design
Design Constraints
Constraints on a
on a
Specialization/Generalization (Cont.)
(Cont.)
Completeness constraint -- specifies whether or not an
entity in the higher-level entity set must belong to at least one
of the lower-level entity sets within a generalization.
total : an entity must belong to one of the lower-level entity
sets
partial: an entity need not belong to one of the lower-level
entity sets

Aggregation
Aggregation
Consider the ternary relationship works_on, which we saw earlier
Suppose we want to record managers for tasks performed by an
employee at a branch

Aggregation
Aggregation (Cont.)
(Cont.)
Relationship sets works_on and manages represent overlapping information
Every manages relationship corresponds to a works_on relationship
However, some works_on relationships may not correspond to any
manages relationships
So we can’t discard the works_on relationship
Eliminate this redundancy via aggregation
Treat relationship as an abstract entity
Allows relationships between relationships
Abstraction of relationship into new entity
Without introducing redundancy, the following diagram represents:
An employee works on a particular job at a particular branch
An employee, branch, job combination may have an associated manager

E
E-
-R Diagram With Aggregation
R Diagram With Aggregation

E
E-
-R Design Decisions
R Design Decisions
The use of an attribute or entity set to represent an object.
Whether a real-world concept is best expressed by an entity set or
a relationship set.
The use of a ternary relationship versus a pair of binary
relationships.
The use of a strong or weak entity set.
The use of specialization/generalization – contributes to modularity
in the design.
The use of aggregation – can treat the aggregate entity set as a
single unit without concern for the details of its internal structure.

E
E-
-R
R
Diagram
Diagram
for a
for a
Banking
Banking
Enterprise
Enterprise

Summary
Summary
of Symbols
of Symbols

Summary of Symbols
Summary of Symbols (Cont.)
(Cont.)

Example 2: COMPANY Database
Example 2: COMPANY Database
Requirements (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.
We store each EMPLOYEE’s social security number,
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
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 John Smith, the Research
DEPARTMENT, the ProductX PROJECT
Attributes are properties used to describe an entity.
For example an EMPLOYEE entity may have a Name, SSN,
Address, Sex, BirthDate
A specific entity will have a value for each of its attributes.
For example a specific employee entity may have Name='John
Smith', SSN='123456789', Address ='731, Fondren, Houston, TX',
Sex='M', BirthDate='09-JAN-55‘
Each attribute has a value set (or data type) associated with it
e.g. integer, string, subrange, enumerated type, …

Types of Attributes
Types of Attributes
Simple
Each entity has a single atomic value for the attribute.
For example, SSN 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, Color of a CAR or PreviousDegrees of a
STUDENT. Denoted as {Color} or {PreviousDegrees}.

Entity Types and Key Attributes
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, SSN of EMPLOYEE.
A key attribute may be composite.
For example, VehicleTagNumber is a key of the CAR entity type
with components (Number, State).
An entity type may have more than one key.
For example, the CAR entity type may have two keys:
VehicleIdentificationNumber (popularly called VIN) and
VehicleTagNumber (Number, State), also known as license_plate number.

Relationships and Relationship Types
A relationship relates two or more distinct entities with a specific
meaning.
For example, EMPLOYEE John Smith works on the ProductX
PROJECT or EMPLOYEE Franklin Wong manages the Research
DEPARTMENT.
Relationships of the same type are grouped 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.
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.

Example relationship instances of the WORKS_FOR relationship bet
Example relationship instances of the WORKS_FOR relationship between
ween
EMPLOYEE and DEPARTMENT
EMPLOYEE and DEPARTMENT
e1
e2
e3
e4
e5
e6
e7
EMPLOYEE
r1
r2
r3
r4
r5
r6
r7
WORKS_FOR
d1
d2
d3
DEPARTMENT

Example relationship instances of the WORKS_ON relationship betw
Example relationship instances of the WORKS_ON relationship between
een
EMPLOYEE and PROJECT
EMPLOYEE and PROJECT
e1
e2
e3
e4
e5
e6
e7
r1
r2
r3
r4
r5
r6
r7
p1
p2
p3
r8
r9

Weak Entity Types
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 dependent’s first
name and birthdates, 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

Constraints on Relationships
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 existence-
dependent)
one or more (mandatory, existence-dependent)

Many
Many-
-to
to-
-one (N:1) RELATIONSHIP
one (N:1) RELATIONSHIP
e1
e2
e3
e4
e5
e6
e7
EMPLOYEE
r1
r2
r3
r4
r5
r6
r7
WORKS_FOR
d1
d2
d3
DEPARTMENT

Many
Many-
-to
to-
-many (M:N) RELATIONSHIP
many (M:N) RELATIONSHIP
e1
e2
e3
e4
e5
e6
e7
r1
r2
r3
r4
r5
r6
r7
p1
p2
p3
r8
r9

Relationships and Relationship Types (2)
(2)
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 relationship type can also 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.

A RECURSIVE RELATIONSHIP SUPERVISION
A RECURSIVE RELATIONSHIP SUPERVISION
e1
e2
e3
e4
e5
e6
e7
EMPLOYEE
r1
r2
r3
r4
r5
r6
SUPERVISION
2
1
1
2
2
1
1
1
2
1
2
2
© The Benjamin/Cummings Publishing Company, Inc. 1994, Elmasri/Navathe, Fundamentals of Database Systems, Second Edition

Structural Constraints
Structural Constraints –
–
one way to express semantics
one way to express semantics
of relationships
of relationships
Structural constraints on relationships:
Cardinality ratio (of a binary relationship): 1:1, 1:N, N:1, or M:N
SHOWN BY PLACING APPROPRIATE NUMBER ON
THE LINK.
Participation constraint (on each participating entity type):
total (called existence dependency) or partial.
SHOWN BY DOUBLE LINING THE LINK
NOTE: These are easy to specify for Binary Relationship Types.

Alternative (min, max) notation for relationship
Alternative (min, max) notation for relationship
structural constraints
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 (0,n) for participation of DEPARTMENT in WORKS_FOR

The (min,max) notation
relationship constraints
(1,1)
(0,1)
(1,N)
(1,1)

Reduction to Relation Schemas
Reduction to Relation Schemas
Primary keys allow entity sets and relationship sets to
be expressed uniformly as relation schemas that
represent the contents of the database.
A database which conforms to an E-R diagram can be
represented by a collection of schemas.
For each entity set and relationship set there is a
unique schema that is assigned the name of the
corresponding entity set or relationship set.
Each schema has a number of columns (generally
corresponding to attributes), which have unique names.

Representing Entity Sets as Schemas
Representing Entity Sets as Schemas
A strong entity set reduces to a schema with the
same attributes.
A weak entity set becomes a table that includes a
column for the primary key of the identifying
strong entity set
payment =
( loan_number, payment_number, payment_date, payment_amount )

Representing Relationship Sets as Schemas
Representing Relationship Sets as Schemas
A many-to-many relationship set is represented as a
schema with attributes for the primary keys of the two
participating entity sets, and any descriptive attributes
of the relationship set.
Example: schema for relationship set borrower
borrower = (customer_id, loan_number )

Redundancy of Schemas
Many-to-one and one-to-many relationship sets that are total on the
many-side can be represented by adding an extra attribute to the
“many” side, containing the primary key of the “one” side
Example: Instead of creating a schema for relationship set
account_branch, add an attribute branch_name to the schema arising
from entity set account

Redundancy of Schemas (Cont.)
(Cont.)
For one-to-one relationship sets, either side can be chosen
to act as the “many” side
That is, extra attribute can be added to either of the
tables corresponding to the two entity sets
If participation is partial on the “many” side, replacing a
schema by an extra attribute in the schema corresponding
to the “many” side could result in null values
The schema corresponding to a relationship set linking a
weak entity set to its identifying strong entity set is
redundant.
Example: The payment schema already contains the
attributes that would appear in the loan_payment
schema (i.e., loan_number and payment_number).

Composite and
Composite and Multivalued
Multivalued Attributes
Attributes
Composite attributes are flattened out by creating a separate attribute for each
component attribute
Example: given entity set customer with composite attribute name with
component attributes first_name and last_name the schema corresponding to the
entity set has two attributes
name.first_name and name.last_name
A multivalued attribute M of an entity E is represented by a separate schema EM
Schema EM has attributes corresponding to the primary key of E and an attribute
corresponding to multivalued attribute M
Example: Multivalued attribute dependent_names of employee is represented by
a schema: employee_dependent_names = ( employee_id, dname)
Each value of the multivalued attribute maps to a separate tuple of the relation
on schema EM
An employee entity with primary key 123-45-6789 and dependents Jack and
Jane maps to two tuples:
(123-45-6789 , Jack) and (123-45-6789 , Jane)

Representing Specialization via Schemas
Representing Specialization via Schemas
Method 1:
Form a schema for the higher-level entity
Form a schema for each lower-level entity set, include primary
key of higher-level entity set and local attributes
schema attributes
person name, street, city
customer name, credit_rating
employee name, salary
Drawback: getting information about, an employee requires
accessing two relations, the one corresponding to the low-level
schema and the one corresponding to the high-level schema

Representing Specialization as Schemas
Representing Specialization as Schemas (Cont.)
(Cont.)
Method 2:
Form a schema for each entity set with all local and inherited attributes
schema attributes
person name, street, city
customer name, street, city, credit_rating
employee name, street, city, salary
If specialization is total, the schema for the generalized entity set (person)
not required to store information
Can be defined as a “view” relation containing union of specialization
relations
But explicit schema may still be needed for foreign key constraints
Drawback: street and city may be stored redundantly for people who are
both customers and employees

Schemas Corresponding to Aggregation
To represent aggregation, create a schema containing
primary key of the aggregated relationship,
the primary key of the associated entity set
any descriptive attributes
For example, to represent aggregation manages between
relationship works_on and entity set manager, create a schema
manages (employee_id, branch_name, title, manager_name)

Schemas Corresponding to Aggregation (Cont.)
(Cont.)
Schema works_on is redundant provided we are willing to store null
values for attribute manager_name in relation on schema manages

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