Database System Concepts, 7th Ed.
©Silberschatz, Korth and Sudarshan
See www.db-book.com for conditions on re-use
Chapter 6: Database Design Using the E-R
Model
©Silberschatz, Korth and Sudarshan6.4Database System Concepts - 7th Edition
Design Phases
▪ Initial phase -- characterize fully the data needs of the prospective
database users.
▪ Second phase -- choosing a data model
• Applying the concepts of the chosen data model
• Translating these requirements into a conceptual schema of the
database.
• A fully developed conceptual schema indicates the functional
requirements of the enterprise.
▪ Describe the kinds of operations (or transactions) that will be
performed on the data.
©Silberschatz, Korth and Sudarshan6.5Database System Concepts - 7th Edition
Design Phases (Cont.)
▪ Final Phase -- Moving from an abstract data model to the implementation
of the database
• Logical Design – Deciding on the database schema.
▪ Database design requires that we find a “good” collection of
relation schemas.
▪ Business decision – What attributes should we record in the
database?
▪ Computer Science decision – What relation schemas should we
have and how should the attributes be distributed among the
various relation schemas?
• Physical Design – Deciding on the physical layout of the database
©Silberschatz, Korth and Sudarshan6.6Database System Concepts - 7th Edition
Design Alternatives
▪ In designing a database schema, we must ensure that we avoid two
major pitfalls:
• Redundancy: a bad design may result in repeat information.
▪ Redundant representation of information may lead to data
inconsistency among the various copies of information
• Incompleteness: a bad design may make certain aspects of the
enterprise difficult or impossible to model.
▪ Avoiding bad designs is not enough. There may be a large number of
good designs from which we must choose.
©Silberschatz, Korth and Sudarshan6.7Database System Concepts - 7th Edition
Design Approaches
▪ Entity Relationship Model (covered in this chapter)
• Models an enterprise as a collection of entities and relationships
▪ Entity: a “thing” or “object” in the enterprise that is distinguishable
from other objects
• Described by a set of attributes
▪ Relationship: an association among several entities
• Represented diagrammatically by an entity-relationship diagram:
▪ Normalization Theory (Next chapter)
• Formalize what designs are bad, and test for them
©Silberschatz, Korth and Sudarshan6.8Database System Concepts - 7th Edition
Outline of the ER Model
©Silberschatz, Korth and Sudarshan6.10Database System Concepts - 7th Edition
Entity Sets
▪ An entity is an object that exists and is distinguishable from other
objects.
• Example: specific person, company, event, plant
▪ 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
▪ An entity is represented by a set of attributes; i.e., descriptive properties
possessed by all members of an entity set.
• Example:
instructor = (ID, name, salary )
course= (course_id, title, credits)
▪ A subset of the attributes form a primary key of the entity set; i.e.,
uniquely identifying each member of the set.
©Silberschatz, Korth and Sudarshan6.12Database System Concepts - 7th Edition
Representing Entity sets in ER Diagram
▪ Entity sets can be represented graphically as follows:
• Rectangles represent entity sets.
• Attributes listed inside entity rectangle
• Underline indicates primary key attributes
©Silberschatz, Korth and Sudarshan6.13Database System Concepts - 7th Edition
Relationship Sets
▪ A relationship is an association among several entities
Example:
44553 (Peltier) advisor 22222 (Einstein)
student entity relationship set instructor entity
▪ A relationship set is a mathematical relation among n  2 entities, each
taken from entity sets
{(e1, e2, … en) | e1  E1, e2  E2, …, en  En}
where (e1, e2, …, en) is a relationship
• Example:
(44553,22222)  advisor
©Silberschatz, Korth and Sudarshan6.14Database System Concepts - 7th Edition
Relationship Sets (Cont.)
▪ Example: we define the relationship set advisor to denote the
associations between students and the instructors who act as their
advisors.
▪ Pictorially, we draw a line between related entities.
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Representing Relationship Sets via ER Diagrams
▪ Diamonds represent relationship sets.
©Silberschatz, Korth and Sudarshan6.16Database System Concepts - 7th Edition
Relationship Sets (Cont.)
▪ An attribute can also be associated with a relationship set.
▪ For instance, the advisor relationship set between entity sets instructor
and student may have the attribute date which tracks when the student
started being associated with the advisor
instructor
student
76766 Crick
Katz
Srinivasan
Kim
Singh
Einstein
45565
10101
98345
76543
22222
98988
12345
00128
76543
44553
Tanaka
Shankar
Zhang
Brown
Aoi
Chavez
Peltier
3 May 2008
10 June 2007
12 June 2006
6 June 2009
30 June 2007
31 May 2007
4 May 2006
76653
23121
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Relationship Sets with Attributes
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Roles
▪ Entity sets of a relationship need not be distinct
• Each occurrence of an entity set plays a “role” in the relationship
▪ The labels “course_id” and “prereq_id” are called roles.
©Silberschatz, Korth and Sudarshan6.19Database System Concepts - 7th Edition
Degree of a Relationship Set
▪ Binary relationship
• involve two entity sets (or degree two).
• most relationship sets in a database system are binary.
▪ Relationships between more than two entity sets are rare. Most
relationships are binary. (More on this later.)
• Example: students work on research projects under the guidance of
an instructor.
• relationship proj_guide is a ternary relationship between instructor,
student, and project
©Silberschatz, Korth and Sudarshan6.20Database System Concepts - 7th Edition
Non-binary Relationship Sets
▪ Most relationship sets are binary
▪ There are occasions when it is more convenient to represent relationships
as non-binary.
▪ E-R Diagram with a Ternary Relationship
©Silberschatz, Korth and Sudarshan6.21Database System Concepts - 7th Edition
Complex Attributes
▪ Attribute types:
• Simple and composite attributes.
• Single-valued and multivalued attributes
▪ Example: multivalued attribute: phone_numbers
• Derived attributes
▪ Can be computed from other attributes
▪ Example: age, given date_of_birth
▪ Domain – the set of permitted values for each attribute
©Silberschatz, Korth and Sudarshan6.22Database System Concepts - 7th Edition
Composite Attributes
▪ Composite attributes allow us to divide attributes into subparts (other
attributes).
name address
first_name middle_initial last_name street city state postal_code
street_number street_name apartment_number
composite
attributes
component
attributes
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Representing Complex Attributes in ER Diagram
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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
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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
©Silberschatz, Korth and Sudarshan6.26Database System Concepts - 7th Edition
Mapping Cardinalities
Many to one Many to many
Note: Some elements in A and B may not be mapped to any
elements in the other set
©Silberschatz, Korth and Sudarshan6.27Database System Concepts - 7th Edition
Representing Cardinality Constraints in ER Diagram
▪ 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 between an instructor and a student :
• A student is associated with at most one instructor via the relationship
advisor
• A student is associated with at most one department via stud_dept
©Silberschatz, Korth and Sudarshan6.28Database System Concepts - 7th Edition
One-to-Many Relationship
▪ one-to-many relationship between an instructor and a student
• an instructor is associated with several (including 0) students via
advisor
• a student is associated with at most one instructor via advisor,
©Silberschatz, Korth and Sudarshan6.29Database System Concepts - 7th Edition
Many-to-One Relationships
▪ In a many-to-one relationship between an instructor and a student,
• an instructor is associated with at most one student via advisor,
• and a student is associated with several (including 0) instructors via
advisor
©Silberschatz, Korth and Sudarshan6.30Database System Concepts - 7th Edition
Many-to-Many Relationship
▪ An instructor is associated with several (possibly 0) students via advisor
▪ A student is associated with several (possibly 0) instructors via advisor
©Silberschatz, Korth and Sudarshan6.31Database System Concepts - 7th Edition
Total and Partial Participation
▪ Total participation (indicated by double line): every entity in the entity set
participates in at least one relationship in the relationship set
participation of student in advisor relation is total
▪ every student must have an associated instructor
▪ Partial participation: some entities may not participate in any relationship
in the relationship set
• Example: participation of instructor in advisor is partial
©Silberschatz, Korth and Sudarshan6.32Database System Concepts - 7th Edition
Notation for Expressing More Complex Constraints
▪ A line may have an associated minimum and maximum cardinality, shown
in the form l..h, where l is the minimum and h the maximum cardinality
• A minimum value of 1 indicates total participation.
• A maximum value of 1 indicates that the entity participates in at most
one relationship
• A maximum value of * indicates no limit.
▪ Example
• Instructor can advise 0 or more students. A student must have 1
advisor; cannot have multiple advisors
©Silberschatz, Korth and Sudarshan6.33Database System Concepts - 7th Edition
Cardinality Constraints on Ternary Relationship
▪ We allow at most one arrow out of a ternary (or greater degree)
relationship to indicate a cardinality constraint
▪ For example, an arrow from proj_guide to instructor indicates each
student has at most one guide for a project
▪ If there is more than one arrow, there are two ways of defining the
meaning.
• For example, 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
©Silberschatz, Korth and Sudarshan6.34Database System Concepts - 7th Edition
Primary Key
▪ Primary keys provide a way to specify how entities and relations are
distinguished. We will consider:
• Entity sets
• Relationship sets.
• Weak entity sets
©Silberschatz, Korth and Sudarshan6.35Database System Concepts - 7th Edition
Primary key for Entity Sets
▪ By definition, individual entities are distinct.
▪ From a database perspective, the differences among them must be
expressed in terms of their attributes.
▪ The values of attribute values of an entity must be such that they can
uniquely identify the entity.
• No two entities in an entity set are allowed to have exactly the same
value for all attributes.
▪ A key for an entity is a set of attributes that suffice to distinguish entities
from each other
©Silberschatz, Korth and Sudarshan6.36Database System Concepts - 7th Edition
Primary Key for Relationship Sets
▪ To distinguish among various relationships in a relationship set we use the
individual primary keys of the entities in the relationship set.
• Let R be a relationship set involving entity sets E1, E2, .. En
• The primary key for R consists of the union of the primary keys of
entity sets E1, E2, ..En
• If the relationship set R has attributes a1, a2, .., am associated with it,
then the primary key of R also includes the attributes a1, a2, .., am
▪ Example: relationship set “advisor”.
• The primary key consists of instructor.ID and student.ID
▪ The choice of the primary key for a relationship set depends on the
mapping cardinality of the relationship set.
©Silberschatz, Korth and Sudarshan6.37Database System Concepts - 7th Edition
Choice of Primary Key for Binary Relationship
▪ Many-to-Many relationships. The preceding union of the primary keys is a
minimal superkey and is chosen as the primary key.
▪ One-to-Many relationships. The primary key of the “Many” side is a minimal
superkey and is used as the primary key.
▪ Many-to-one relationships. The primary key of the “Many” side is a minimal
superkey and is used as the primary key.
▪ One-to-one relationships. The primary key of either one of the participating
entity sets forms a minimal superkey, and either one can be chosen as the
primary key.
©Silberschatz, Korth and Sudarshan6.41Database System Concepts - 7th Edition
Expressing Weak Entity Sets
▪ In E-R diagrams, a weak entity set is depicted via a double rectangle.
▪ We underline the discriminator of a weak entity set with a dashed line.
▪ The relationship set connecting the weak entity set to the identifying
strong entity set is depicted by a double diamond.
▪ Primary key for section – (course_id, sec_id, semester, year)
©Silberschatz, Korth and Sudarshan6.42Database System Concepts - 7th Edition
Redundant Attributes
▪ Suppose we have entity sets:
• student, with attributes: ID, name, tot_cred, dept_name
• department, with attributes: dept_name, building, budget
▪ We model the fact that each student has an associated department using
a relationship set stud_dept
▪ The attribute dept_name in student below replicates information present
in the relationship and is therefore redundant
• and needs to be removed.
▪ BUT: when converting back to tables, in some cases the attribute gets
reintroduced, as we will see later.
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E-R Diagram for a University Enterprise
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Reduction to Relation Schemas
©Silberschatz, Korth and Sudarshan6.45Database System Concepts - 7th Edition
Reduction to Relation Schemas
▪ Entity sets and relationship sets can be expressed uniformly as relation
schemas that represent the contents of the database.
▪ A database that 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.
©Silberschatz, Korth and Sudarshan6.46Database System Concepts - 7th Edition
Representing Entity Sets
▪ A strong entity set reduces to a schema with the same attributes
student(ID, name, tot_cred)
▪ A weak entity set becomes a table that includes a column for the primary
key of the identifying strong entity set
section ( course_id, sec_id, sem, year )
▪ Example
©Silberschatz, Korth and Sudarshan6.47Database System Concepts - 7th Edition
Representation of Entity Sets with Composite Attributes
▪ Composite attributes are flattened out by creating a
separate attribute for each component attribute
• Example: given entity set instructor 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
▪ Prefix omitted if there is no ambiguity
(name_first_name could be first_name)
▪ Ignoring multivalued attributes, extended instructor
schema is
• instructor(ID,
first_name, middle_initial, last_name,
street_number, street_name,
apt_number, city, state, zip_code,
date_of_birth)
©Silberschatz, Korth and Sudarshan6.48Database System Concepts - 7th Edition
Representation of Entity Sets with Multivalued Attributes
▪ 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 the multivalued attribute M
▪ Example: Multivalued attribute phone_number of instructor is
represented by a schema:
inst_phone= ( ID, phone_number)
▪ Each value of the multivalued attribute maps to a separate tuple of the
relation on schema EM
• For example, an instructor entity with primary key 22222 and phone
numbers 456-7890 and 123-4567 maps to two tuples:
(22222, 456-7890) and (22222, 123-4567)
©Silberschatz, Korth and Sudarshan6.49Database System Concepts - 7th Edition
Representing Relationship Sets
▪ 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 advisor
advisor = (s_id, i_id)
©Silberschatz, Korth and Sudarshan6.50Database System Concepts - 7th Edition
Redundancy of Schemas
▪ Many-to-one and one-to-many relationship sets that are total on the manyside
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 inst_dept, add
an attribute dept_name to the schema arising from entity set instructor
▪ Example
©Silberschatz, Korth and Sudarshan6.51Database System Concepts - 7th Edition
Redundancy of Schemas (Cont.)
▪ For one-to-one relationship sets, either side can be chosen to act as the
“many” side
• That is, an 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
©Silberschatz, Korth and Sudarshan6.52Database System Concepts - 7th Edition
Redundancy of Schemas (Cont.)
▪ The schema corresponding to a relationship set linking a weak entity set
to its identifying strong entity set is redundant.
▪ Example: The section schema already contains the attributes that would
appear in the sec_course schema
©Silberschatz, Korth and Sudarshan6.53Database System Concepts - 7th Edition
Extended E-R Features
©Silberschatz, Korth and Sudarshan6.54Database System Concepts - 7th Edition
Specialization
▪ Top-down design process; we designate sub-groupings within an entity set
that are distinctive from other entities in the set.
▪ These sub-groupings 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., instructor “is a”
person).
▪ Attribute inheritance – a lower-level entity set inherits all the attributes
and relationship participations of the higher-level entity set to which it is
linked.
©Silberschatz, Korth and Sudarshan6.55Database System Concepts - 7th Edition
Specialization Example
▪ Overlapping – employee and student
▪ Disjoint – instructor and secretary
▪ Total and partial
©Silberschatz, Korth and Sudarshan6.56Database System Concepts - 7th Edition
Representing Specialization via Schemas
▪ Method 1:
• Form a schema for the higher-level entity
• Form a schema for each lower-level entity set, including the primary
key of the higher-level entity set and local attributes
• 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
©Silberschatz, Korth and Sudarshan6.57Database System Concepts - 7th Edition
Representing Specialization as Schemas (Cont.)
▪ Method 2:
• Form a schema for each entity set with all local and inherited
attributes
• Drawback: name, street, and city may be stored redundantly for
people who are both students and employees
©Silberschatz, Korth and Sudarshan6.58Database System Concepts - 7th Edition
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.
©Silberschatz, Korth and Sudarshan6.59Database System Concepts - 7th Edition
Completeness constraint
▪ 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 any of the lower-level entity
sets
©Silberschatz, Korth and Sudarshan6.60Database System Concepts - 7th Edition
Aggregation
▪ Consider the ternary relationship proj_guide, which we saw earlier
▪ Suppose we want to record evaluations of a student by a guide on a
project
©Silberschatz, Korth and Sudarshan6.61Database System Concepts - 7th Edition
Aggregation (Cont.)
▪ Relationship sets eval_for and proj_guide represent overlapping
information
• Every eval_for relationship corresponds to a proj_guide relationship
• However, some proj_guide relationships may not correspond to any
eval_for relationships
▪ So we can’t discard the proj_guide relationship
▪ Eliminate this redundancy via aggregation
• Treat relationship as an abstract entity
• Allows relationships between relationships
• Abstraction of relationship into new entity
©Silberschatz, Korth and Sudarshan6.62Database System Concepts - 7th Edition
Aggregation (Cont.)
▪ Eliminate this redundancy via aggregation without introducing
redundancy, the following diagram represents:
• A student is guided by a particular instructor on a particular project
• A student, instructor, project combination may have an associated
evaluation
©Silberschatz, Korth and Sudarshan6.63Database System Concepts - 7th Edition
Reduction to Relational Schemas
▪ To represent aggregation, create a schema containing
• Primary key of the aggregated relationship,
• The primary key of the associated entity set
• Any descriptive attributes
▪ In our example:
• The schema eval_for is:
eval_for (s_ID, project_id, i_ID, evaluation_id)
• The schema proj_guide is redundant.
©Silberschatz, Korth and Sudarshan6.64Database System Concepts - 7th Edition
Design Issues
©Silberschatz, Korth and Sudarshan6.65Database System Concepts - 7th Edition
Entities vs. Attributes
▪ Use of entity sets vs. attributes
▪ Use of phone as an entity allows extra information about phone numbers
(plus multiple phone numbers)
©Silberschatz, Korth and Sudarshan6.66Database System Concepts - 7th Edition
Entities vs. Relationship sets
▪ Use of entity sets vs. relationship sets
A possible guideline is to designate a relationship set to describe
an action that occurs between entities
▪ Placement of relationship attributes
For example, attribute date as attribute of advisor or as attribute
of student
©Silberschatz, Korth and Sudarshan6.67Database System Concepts - 7th Edition
Binary Vs. Non-Binary Relationships
▪ Although it is possible to replace any non-binary (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.
▪ Some relationships that appear to be non-binary may be better
represented using binary relationships
• For example, a ternary relationship parents, relating a child to
his/her father and mother, is best replaced by two binary
relationships, father and mother
▪ Using two binary relationships allows partial information (e.g.,
only the mother being known)
• But there are some relationships that are naturally non-binary
▪ Example: proj_guide
©Silberschatz, Korth and Sudarshan6.68Database System Concepts - 7th Edition
Converting Non-Binary Relationships to Binary Form
▪ In general, any non-binary relationship can be represented using binary
relationships by creating an artificial entity set.
• Replace R between entity sets A, B and C by an entity set E, and three
relationship sets:
1. RA, relating E and A 2. RB, relating E and B
3. RC, relating E and C
• Create an identifying attribute for E and add any attributes of R to E
• For each relationship (ai , bi , ci) in R, create
1. a new entity ei in the entity set E 2. add (ei , ai ) to RA
3. add (ei , bi ) to RB 4. add (ei , ci ) to RC
©Silberschatz, Korth and Sudarshan6.69Database System Concepts - 7th Edition
E-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.
©Silberschatz, Korth and Sudarshan6.70Database System Concepts - 7th Edition
Summary of Symbols Used in E-R Notation
©Silberschatz, Korth and Sudarshan6.71Database System Concepts - 7th Edition
Symbols Used in E-R Notation (Cont.)
©Silberschatz, Korth and Sudarshan6.72Database System Concepts - 7th Edition
End of Chapter 6