3. History of E-R Model
► E-R Model was proposed
by Dr. Peter Chen
(currently professor at
Louisiana State
University)
► Chen’s original paper on
E-R Model is the 35th
most sited paper in
computer science
► Chen has written papers
interconnecting E-R
model and linguistics
4. Introduction
►Database Structure is often called Database
Schema
►E-R model is graphical in nature, thus
making it easy to analyze and observe
relationship between data elements
►Most DBMS are based upon E-R model
►E-R model is not a good match for the
sophisticated data structures required in
today’s DBMS
5. Elements of E-R Model
►Data represented graphically via entity-
relationship diagram which contains 3 main
element types:
Entity sets
Attributes
Relationships
6. Entity sets, Attributes, Relationships
► Entity set
Is an abstract object, collection of such objects forms
an entity set.
Similar notion as in OO design
► Attribute:
Some concrete data (or object type) by which entity set
is defined
► Relationship
Specific connection between 2 or more entity sets
7. E-R Diagram
►Represents E-R elements by nodes of
specific shape to indicate kind
Entity sets are represented by rectangles
Attributes are shown as ovals
Relationships correspond to diamonds
►Simple example from the book:
9. Instances of E-R diagram
►DB described by E-R will contain specific
data (i.e. database instance)
►Each entity set will contain a particular finite
set of entities
►Each entity contains a particular value for
each attribute
►E-R data is not stored directly in DB
10. E-R Relationships
► Suppose R is a relationship connecting entity sets
E and F. Then:
If each member of E can be connected by R to at
most one member of F, then we say that R is many-
one from E to F. Note that in a many-one relationship
from E to F, each entity in F can be connected to many
members of E.
If R is both many-one from E to F and many-one from F
to E, then we say that R is one-one. In a one-one
relationship an entity of either entity set can be
connected to at most one entity of the other set.
If R is neither many-one from E to F or from F to E,
then we say R is many-many
11. Multi-way relationships
► There is a relationship Sequel-of between the entity set
Movies and itself.
► To differentiate the two movies in a relationship, one line
is labeled by the role Original and one by the role Sequel,
indicating the original movie and its sequel, respectively.
12. Relationships Continued
►Some data models limit relationships to be
binary
►It is possible to convert multi-way
relationship into a collection of binary many-
one relationships
►Need to introduce a connecting entity set,
which will act as a bridge between smaller
sets which come from splitting a larger
multi-way relationship set.
►Connecting entity set might have its own
attributes
13. Design Principles
► Faithfulness:
Design has to comply strictly with specifications
Logical attributes and relationships
► Avoid redundancy
► "Everything should be made as simple as possible,
but not simpler.” Albert Einstein
► Choose right relationships
► Select right elements
Many choices exist between using attributes and using
entity set/relationship combinations
An attribute is simpler to implement than either entity
set or a relationship
14. Design Principals Cont.
►To replace an entity set by an attribute or
attributes of several entity sets 3 conditions
must be enforced:
All relationships in which entity set is involved
must have arrows entering it. That is, it must be
the “one” in many-one relationships, or its
generalization for the case of multi-way
relationships.
The attributes for E must collectively identify an
entity. if there are several attributes, then no
attribute must depend on the other attributes
No relationship involves E more than once
