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Entity-Relationship Data
Model
CS 157A
Professor Sin-Min Lee
Student: Yen-Chu Pan

Elements of E-R Model
 In the E/R model, the structure of data is
represented graphically, as an “entity-
relationship diagram,” using three Principal
element types:
 Entity Sets.
 An entity is an abstract object of some sort, and a
collection of similar entities forms an entity set.
 Attributes
 The properties of the entities in the set.
 Relationship
 The connections among two or more entity Sets

Example of Elements of E-R
Model
 Entity Sets
 Departments
 Professors
 Students
 Administrators
 Attributes
 Name of Departments, Phone No., Address...
 Name, SSN, Address of Professors...
 Relationship
 Students and Professors are under a certain
department
 Admin manage the campus/ departments

Example of the 3 elements in
E/R Diagram

Classification of Constraints
1. Keys
2. Single-value constraints
3. Multi-valued constraints
4. Mapping Cardinalities and
Participation Constraints

Key in the E/R Model
 Superkey is a set of one or more attributes that, taken
collectively, for us to identify uniquely an item in the entity set.
For example, customer-id is a superkey.
 Candidate key is a minimal superkey. For example,
customer-name and customer-street is sufficient to distinguish
among members of the customer entity set. Then {customer-
name, customer-street } is a candidate key.
 Primary key denotes a candidate key that is chosen by the
database designer as the principal means of identifying items
within an entity set. the primary key should be chosen such
that its attributes are never, or very rarely, changed. For
example, Social-security numbers are guaranteed to never
changed.

Single/Multi-valued attributes
 Single-valued attributes are attributes that
only have a single value for a particular
entity.
 Multi-valued attributes refers to items that
are not singled-value and Null valued. For
example, consider an employee entity set
with the attribute phone-number. An
employee may have zero, one, or several
phone numbers; different employee may
have different numbers of phones.

Mapping Cardinalities or
Cardinality ratios
 Express the number of items to which another item
can be associated via a relationship set
 Are most useful in describing binary relationship
sets. For a binary relationship set R between entity
sets A and B, the mapping cardinality must be one
of the following:
 One to One
 One to Many
 Many to One
 Many to Many

Participation Constraints
 The participation of an entity set E in a
relationship set R is said to be total, if
every item in E participates in at least
one relationship in R. If only some
items in E participate in relationship R,
the participation of entity set E in
relationship R is said to be partial.

Weak Entity Sets
 There is an occasional condition in
which an entity set’s key is composed
of attributes some or all of which
belong to another entity set. Such an
entity set is called a weak entity set.

Discriminator
 The discriminator of a weak entity set is a
set of attributes that allows this distinction to
be made. For example, the discriminator of
a weak entity set payment is the attribute
payment-number, since, for each loan a
payment number uniquely identifies one
single payment for that loan. The
discriminator of a weak entity set is also
called the partial key of the entity set.

Requirements for Weak Entity
Sets
 We cannot obtain key attributes for a weak
entity set indiscriminately. Rather, if E is a
weak entity set then its key consists of:
 Zero or more of its own attributes, and
 Key attributes from entity sets that are reached
by certain many-one relationship from E to other
entity sets. These many-one relationship are
called supporting relationships for E.

Discriminator (cont.)
 Note: although each payment entity is
distinct, payments for different loans may
share the same payment-number. Thus,
payment entity set does not have a primary
key; it is a weak entity set.
 The primary key of a weak entity set is
formed by the primary key of the identifying
entity set, plus the weak entity set’s
discriminator.

Identifying a Weak Entity Type
 A Weak entity type doesn’t have a primary key.
 If X is a weak entity type and Y is the entity type
on which X is dependent.
 We form a primary key for X by combining the
primary key of Y which one or more attributes,
called discriminator or partial key, from X.
 In an E/R Diagram, a partial key is usually dash-
underlined.
 e.g., primary key for DEPENDENT: {Employee
No., DName}.

* Doted-line = double-line
*

References
 Peter Chen’s website:
http://bit.csc.lsu.edu/~chen/chen.html
 Database Systems: A First Course, J.D.
Ullman & J. Widom
 http://www-
db.stanford.edu/~ullman/fcdb.html
 http://www.cs.sjsu.edu/faculty/lee/cs157/25S
p157AL4.ppt
 http://www.cs.sjsu.edu/faculty/lee/cs157/25S
p157AL5Enhanced%20ER-diagram.ppt

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