This document discusses reducing an ER diagram to relational schemas. It covers issues in ER diagram design like using entity sets vs attributes. It also describes how to represent different ER constructs in relational schemas, such as strong entities with simple/complex attributes, weak entities, and different relationship sets. Relationship sets linking weak entities or with total participation may result in redundant or combined schemas. The overall goal is to produce a quality set of relational schemas from a well-designed ER diagram.

1. REDUCTION TO
RELATIONAL SCHEMAS

2. Summary of Symbols used in ER
Notation

3. Contd..

4. Contd..

5. ER Diagram Design Issues
• Use of Entity Sets v/s Attributes
• Use of Entity Sets v/s Relationship Sets
• Binary v/s n-ary relationships
• Placement of Relationship Attributes

6. Use of Entity Sets v/s Attributes

7. Use of Entity Sets v/s Relationship
Sets
(a)
(b)
One possible guideline in determining
whether to use an entity set or a
relationship set is to designate a
relationship set to describe an action
that occurs between entities.

8. Binary v/s n-ary relationships
Ternary relationship versus three binary
relationships

9. Placement of Relationship
Attributes
• The design decision of where to place descriptive
attributes for a relationship or for an entity should
reflect the characteristics of the enterprise being
modeled.

11. ER
Diagram
for
a
University
Enterprise

12. Reduction to Relational Schemas

13. Introduction
• A conceptual ER model can be reduced to relation
schemas because the two models employ similar design
principles.
• The quality of the resulting set of relation schemas
depends on the quality of the original ER diagram.
• Entity sets and relationship sets can be expressed
uniformly as relation schemas with the name of the
corresponding entity set or relationship set.
• Each schema has a number of columns (attributes),
which have unique names.

14. Example
Entity Set Relational Schema
Employees
ssn
name
lot
Employees(ssn, name, lot)
Create table Employees (
ssn number(5),
name varchar (10),
lot number(5),
primary key(ssn));

15. Example

16. Representation of Strong Entity
Sets with Simple Attributes
• A strong entity set reduces to a schema with the
same attributes
• Example: course(course_id, title, credits)
• For schemas derived from strong entity sets, the
primary key of the entity set serves as the primary
key of the resulting schema.

18. Representation of Strong Entity
Sets with Complex Attributes
• Complex Attributes – Composite, Multi-valued and
Derived Attributes.
• Represent composite attributes by creating a
separate attribute for each of the component
attributes(sub-parts).
• Represent multi-valued attributes itself as a separate
schema.
• Derived attributes are not explicitly represented in
relational model.

19. Representation of Composite
Attributes

20. Representation of Multivalued Attributes

21. Example
Instructor
(ID, first_name, middle_initial,
last_name, street_number,
street_name, apt_number, city, state,
zip_code, date_of_birth)

22. Representation of Weak Entity Sets
• For schemas derived from a weak entity set, the
combination of the primary key of the strong entity
set and the discriminator of the weak entity set
serves as the primary key of the schema.
• Example : section ( course_id, sec_id, sem, year )

24. Representation of Relationship Sets
• Binary many-to-many relationship set is represented
as a schema with the union of primary keys of the
two participating entity sets.
• Example: schema for relationship set advisor
advisor = (s_id, i_id)

25. Contd..
• Binary one-to-one relationship set is represented as a
schema with the primary key of any one of the two
participating entity sets.
• Binary many-to-one or one-to-many relationship set
is represented as a schema with primary key of the
entity set on the “many” side.

26. Contd..
• N-ary relationship set:
 Without any arrows: represented as a schema
with union of primary key attributes.
 With arrows on one side: represented as a schema
with primary key of the entity set not on the
“arrow” side.

27. For one-to-one relationship without total
participation

28. For one-to-one relationship with one entity
set having total participation

29. For one-to-many/many-to-one relationship with
one entity set having total participation on “many”
side

30. For many-to-many relationship

31. Redundancy of Schemas
• The schema corresponding to a relationship set
linking a weak entity set to its identifying entity set
is redundant.
• In the example E-R diagram, the weak entity set
section is dependent on the strong entity set course
via the relationship set sec course.

32. Contd..
• The primary key of section is {course id, sec id, semester,
year} and the primary key of course is course id.
• Since sec_course has no descriptive attributes, the
sec_course schema has attributes {course id, sec id,
semester, year} as the primary key
• Thus, the sec_course schema is redundant.
{course id, sec id, semester, year}
{course id, title, credits}
{course id, sec id, semester, year}

33. Combination of Schemas
• Consider a many-to-one relationship set AB from
entity set A to entity set B.
• Resulting schemas: A, B, and AB
• If the participation of A in the relationship is total;
i.e., every entity A in the entity set B must
participate in the relationship AB, then
– Combine the schemas A and AB to form a single
schema consisting of the union of attributes of
both schemas.
• The primary key of the combined schema is the
primary key of the entity set into whose schema the
relationship set schema was merged.

34. Contd..
• 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
inst_dept, add an attribute dept_name to the schema arising
from entity set instructor

35. MIND MAP

36. • ER Diagram Design Issues
– Use of Entity Sets v/s Attributes
– Use of Entity Sets v/s Relationship Sets
– Binary v/s n-ary relationships
– Placement of Relationship Attributes
REDUCTION TO RELATIONAL SCHEMA
• Representation of Strong Entity Sets with simple
attributes
• Representation of Strong Entity Sets with complex
attributes
o Composite attributes
o Multi-valued attributes
o Derived attributes
• Representation of Weak Entity Sets
• Representation of Relationship Sets
o Redundancy of Schemas
o Combination of Schemas
Summary