ER Modeling and Introduction to RDBMS

Data Base Management System
Unit - 3
ER Modeling and Introduction to RDBMS
Date:
Presented By:
Rubal Sagwal
Department of Computer Engineering
2Rubal

Contents
• Entity type
• Entity sets
• Attributes and keys
• The ER Model
• ER Diagram & Database design with the ER Model
• Introduction
• Relational Model
• Concepts
• Characteristics
3

Introduction
Entity Types – Entity Sets – Attributes and Keys
4

Introduction
• A database can be modeled as:
• A collection of entities,
• Relationship among entities.
5

Introduction
• An Entity–relationship model (ER model) describes
the structure of a database with the help of a
diagram, which is known as Entity Relationship
Diagram (ER Diagram).
• An ER model is a design or blueprint of a database
that can later be implemented as a database.
• The main components of E-R model are: entity set
and relationship set.
6

Introduction
• An ER diagram shows the relationship among
entity sets.
• An entity set is a group of similar entities and
these entities can have attributes.
• In terms of DBMS, an entity is a table or attribute
of a table in database, so by showing relationship
among tables and their attributes, ER diagram
shows the complete logical structure of a database.
7

Uses of entity relationship diagrams
• Database design: ER diagrams are used to model
and design relational databases, in terms of logic
and business rules (in a logical data model) and in
terms of the specific technology to be implemented
(in a physical data model.) In software engineering,
an ER diagram is often an initial step in determining
requirements for an information systems project.
• Database troubleshooting: ER diagrams are used to
analyze existing databases to find and resolve
problems in logic or deployment. Drawing the
diagram should reveal where it’s going wrong.
8

Uses of entity relationship diagrams
• Business information systems: The diagrams are
used to design or analyze relational databases used
in business processes.
• Education: Databases are today’s method of storing
relational information for educational purposes and
later retrieval, so ER Diagrams can be valuable in
planning those data structures.
• Research: Since so much research focuses on
structured data, ER diagrams can play a key role in
setting up useful databases to analyze the data.
9

Entity
• An entity is an object that exists and is
distinguishable from other objects.
• Anything about which data are to be collected and
stored.
• Example: specific person, company, event, plant
• An entity is represented as rectangle in an ER
diagram.
10

Weak - Entity
• An entity that cannot be uniquely identified by its
own attributes and relies on the relationship with
other entity is called weak entity.
• The weak entity is represented by a double
rectangle.
• For example – a bank account cannot be uniquely
identified without knowing the bank to which the
account belongs, so bank account is a weak entity.
• A Engineering branch cannot be identified without
knowing the College/ University to which branch
belongs.
11

Weak - Entity
12
College Branch
Bank Account
College
Bank

Attribute
• An attribute describes the property of an entity.
• A characteristic of an entity.
• An attribute is represented as Oval in an ER
diagram.
• Entities have attributes.
• Each entity has a value for each of its attributes
known as a Domain.
• Example: people have names and addresses
13

Attribute
• Example:
customer = (customer_id, customer_name,
customer_street, customer_city )
loan = (loan_number, amount )
• Domain – the set of permitted values for each attribute.
• Attribute types:
1. Key Attribute
2. composite attributes.
3. Single-valued and multi-valued attributes
• Example: multivalued attribute: phone_numbers
4. Derived attributes
• Can be computed from other attributes
• Example: age, given date_of_birth
14

Attribute Types
1. Key attribute: can uniquely identify an entity
from an entity set.
• For example, student roll number can uniquely
identify a student from a set of students.
• Key attribute is represented by oval same as other
attributes however the text of key attribute is
underlined.
15
Roll Number

Attribute Types
2. Composite Attribute: An attribute that is a
combination of other attributes is known as
composite attribute.
• For example: In student entity, the student address
is a composite attribute as an address is composed
of other attributes such as pin code, state, country.
16
Name
Middle
Name
Last
Name
First
Name

Attribute Types
3. Multivalued Attribute: An attribute that can hold
multiple values is known as multivalued attribute.
• It is represented with double ovals in an ER
Diagram.
• For example – A person can have more than one
phone numbers so the phone number attribute is
multivalued.
17

Attribute Types
4. Derived attributes: derived attribute is one whose
value is dynamic and derived from another
attribute.
• It is represented by dashed oval in an ER Diagram.
• For example – Person age is a derived attribute as
it changes over time and can be derived from
another attribute (Date of birth).
18

Entity Set
• 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.
19
E1
E2
E3

Relationship Set
• A relationship is an association among several
entities.
• 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.
21

Relationship Set
22
Teacher SubjectTeaches

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.
• Relationships between more than two entity sets
are rare. Most relationships are binary.
23

Degree of a Relationship Set
• Relationship sets may involve more than two
entity sets.
• 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
24

Mapping Constraints
1. One to One
2. One to Many
3. Many to One
4. Many to Many
25

Mapping Constraints
• Mapping Cardinalities: express the number of
entities to which another entity can be associated
via a relationship.
• For binary relationship sets between entity sets A
and B, the mapping cardinality must be one of:
• One to one (1:1)
• One to many (1:n)
• Many to one (n:1)
• Many to many (n:n)
26

Mapping Constraints
1. One to One: An entity in A is associated with at
most one entity in B, and an entity in B is associated
with at most one entity in A.
27

Mapping Constraints
2. One to Many: An entity in A is associated with any
number (zero or more) of entities in B. An entity in B,
however, can be associated with at most one entity
in A.
28

Mapping Constraints
3. Many to One: An entity in A is associated with at
most one entity in B. An entity in B, however, can be
associated with any number (zero or more) of
entities in A.
29

Mapping Constraints
4. Many to Many: An entity in A is associated with
any number (zero or more)of entities in B, and an
entity in B is associated with any number (zero or
more) of entities in A.
30

Keys
32
• We must have a way to specify how entities within
a given entity set are distinguished.
• A key allows us to identify a set of attributes that
is enough to distinguish entities from each other.
• They are used to establish and identify
relationships between tables and also to uniquely
identify any record or row of data inside a table.
• A Key can be a single attribute or a group of
attributes, where the combination may act as a
key.

Types of Keys
33
1. Primary Key
2. Super key
3. Candidate Key
4. Composite Key
5. Secondary Key
6. Foreign Key

Types of Keys
34
1. Candidate Key:
• The minimal set of attribute which can uniquely identify a
tuple is known as candidate key.
• For Example, STUD_NO in STUDENT relation.
• The value of Candidate Key is unique and not-null (Can’t be
empty) for every tuple.
• There can be more than one candidate key in a relation.
• For Example, STUD_NO as well as STUD_PHONE both are candidate
keys for relation STUDENT.
• The candidate key can be simple (having only one attribute)
or composite as well.
• For Example, {STUD_NO, COURSE_NO} is a composite candidate key
for relation STUDENT_COURSE.

Types of Keys
36
2. Primary Key:
• Primary key is a candidate key that is most
appropriate to become the main key for any table.
• The primary key is selected from one of the
candidate keys and becomes the identifying key of
a table.
• It can uniquely identify any data row of the table.
• Unique + Not Null
• For the table Student we can make the student_id
column as the primary key.

Types of Keys
37
3. Super Key:
• A super key is a set of one of more columns
(attributes) to uniquely identify rows in a table.
• Super Key is a superset of Candidate key.
• In the table defined above super key would include
student_id, (student_id, name), phone etc.
• Confused?
• The first one is pretty simple as student_id is unique for
every row of data, hence it can be used to identity each
row uniquely.

Types of Keys
38
3. Super Key:
• Next comes, (student_id, name), now name of two
students can be same, but their student_id can't be
same hence this combination can also be a key.
• Similarly, phone number for every student will be
unique, hence again, phone can also be a key.
• So they all are super keys.

Types of Keys
39
4. Composite Key:
• Key that consists of two or more attributes that
uniquely identify any record in a table is
called Composite key.
• But the attributes which together form
the Composite key are not a key independently or
individually.

Types of Keys
40
4. Composite Key:

Types of Keys
41
5. Secondary Key / Alternate Key: Only one of the
candidate keys is selected as the primary key. The
rest of them are known as secondary keys.

Types of Keys
42
6. Foreign Key:
• A foreign key is an attribute value in a table that acts as
the primary key in another table. Hence, the foreign
key is useful in linking together two tables.
• Primary Key of any given relation, Foreign Key can be
NULL as well as may contain duplicate tuples i.e. it need
not follow uniqueness constraint.
• For Example, STUD_NO in STUDENT_COURSE relation is
not unique. It has been repeated for the first and third
tuple. However, the STUD_NO in STUDENT relation is a
primary key and it needs to be always unique and it
cannot be null.

E-R Diagram
45
• As we know The entity-relationship (E-R) data model is
based on a perception of a real world that consists of a
collection of basic objects, called entities, and of
relationships among these objects.
• E-R diagram can express the overall logical structure of a
database graphically. E-R diagrams are simple and clear—
qualities that may well account in large part for the
widespread use of the E-R model.

E-R Diagram
46
Such a diagram consists of the following major components:
• Rectangles: which represent entity sets
• Ellipses/ Oval: which represent attributes
• Double ellipses: which represent multivalued attributes
• Dashed ellipses: which denote derived attributes
• Diamonds: which represent relationship sets
• Lines: which link attributes to entity sets and entity sets to
relationship sets
• Double lines: which indicate total participation of an entity
in a relationship set
• Double rectangles: which represent weak entity sets

E-R Diagram – Example
E-R diagram with composite, multivalued, and derived
attributes
48

E-R Diagram – Example
E-R diagram with Ternary relationship set
49

Roles
50
• 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.
• The labels “manager” and “worker” are called
roles; they specify how employee entities interact
via the works_for relationship set.

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.
51

• 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
52

• Many-to-Many:
• A customer is associated with several (possibly 0) loans
via borrower
• A loan is associated with several (possibly 0) customers
via borrower
53

• One-to-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.
54

• Many-to-One:
• 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
55

ER Model: Generalization,
Specialization and Aggregation
56

Generalization
• Generalization is the process of extracting common
properties from a set of entities and create a
generalized entity from it.
• It is a bottom-up approach in which two or more
entities can be generalized to a higher level entity if
they have some attributes in common.
• For Example, STUDENT and FACULTY can be generalized to a
higher level entity called PERSON as shown in next slide.
• In this case, common attributes like P_NAME, P_ADD become
part of higher entity (PERSON) and specialized attributes like
S_FEE become part of specialized entity (STUDENT).
57

Specialization
• In specialization, an entity is divided into sub-
entities based on their characteristics.
• It is a top-down approach where higher level entity
is specialized into two or more lower level entities.
• For Example, EMPLOYEE entity in an Employee
management system can be specialized into DEVELOPER,
TESTER etc. as shown in next slide.
• In this case, common attributes like E_NAME, E_SAL etc.
become part of higher entity (EMPLOYEE) and
specialized attributes like TES_TYPE become part of
specialized entity (TESTER).
59

Aggregation
• An ER diagram is not capable of representing
relationship between an entity and a relationship which
may be required in some scenarios.
• In those cases, a relationship with its corresponding
entities is aggregated into a higher level entity.
• For Example, Employee working for a project may require
some machinery. So, REQUIRE relationship is needed
between relationship WORKS_FOR and entity MACHINERY.
• Using aggregation, WORKS_FOR relationship with its entities
EMPLOYEE and PROJECT is aggregated into single entity and
relationship REQUIRE is created between aggregated entity
and MACHINERY.
61

Relational Model – Introduction
64
• The relational model represents the database as a
collection of relations.
• Informally, each relation resembles a table of values or,
to some extent, a flat file of records. It is called a flat
file because each record has a simple linear or flat
structure.
• When a relation is thought of as a table of values –
each row in the table represents a collection of related
data values.
• A row represents a fact that typically corresponds to a
real-world entity or relationship.
• The table name and column names are used to help to
interpret the meaning of the values in each row.

65
• For example, this table is called STUDENT because
each row represents facts about a particular
student entity.
• Student Table:
USN Name Year Class
12CT12 ABC 1 CTIS
12CT15 CDE 1 CTIS
13BT13 XYZ 2 DS
13BT13 HDG 3 CSE
13BT56 HJL 3 CSE

66
• The column names—USN, Name, year, and Class —
specify how to interpret the data values in each row,
based on the column each value is in.
• All values in a column are of the same data type.
• In the formal relational model terminology –
• A row is called a tuple
• A column header is called an attribute, and
• The table is called a relation.
• The data type describing the types of values that can
appear in each column is represented by a domain of
possible values.

67
• The relational model is today the primary data
model for commercial data-processing applications.
It has attained its primary position because of its
simplicity, which eases the job of the programmer,
as compared to earlier data models such as the
network model or the hierarchical model.
• Relational Model represents how data is stored in
Relational Databases. A relational database
stores data in the form of relations (tables).

Relational Model – Concepts and Characterstics
68
1. Attribute: Attributes are the properties that define a
relation. e.g.; ROLL_NO, NAME
2. Relation Schema: A relation schema represents
name of the relation with its attributes. e.g.;
STUDENT (ROLL_NO, NAME, ADDRESS, PHONE and
AGE) is relation schema for STUDENT. If a schema has
more than 1 relation, it is called Relational Schema.
3. Tuple: Each row in the relation is known as tuple.
4. Relation Instance: The set of tuples of a relation at a
particular instance of time is called as relation
instance.

Relational Model – Concepts
69
5. Degree: The number of attributes in the relation
is known as degree of the relation.
The STUDENT relation defined above has degree 4.
6. Cardinality: The number of tuples in a relation is
known as cardinality. The STUDENT relation defined
above has cardinality 5.
7. Column: Column represents the set of values for a
particular attribute.
8. NULL Values: The value which is not known or
unavailable is called NULL value.

Constraints in Relational Model
70
• Domain Constraints: These are attribute level
constraints. An attribute can only take values which lie
inside the domain range. e.g,; If a constrains AGE>0 is
applied on STUDENT relation, inserting negative value
of AGE will result in failure.
• Key Integrity: Every relation in the database should
have atleast one set of attributes which defines a tuple
uniquely. Those set of attributes is called key. e.g.; USN
in STUDENT is a key. No two students can have same
roll number. So a key has two properties:
• It should be unique for all tuples.
• It can’t have NULL values.

Constraints in Relational Model
71
• Referential Integrity: When one attribute of a
relation can only take values from other attribute of
same relation or any other relation, it is called
referential integrity.

Relational Model – Advantages
72
• Structural independence: In relational model, changes in the
database structure do not affect the data access. When it is
possible to make change to the database structure without
affecting the DBMS's capability to access data, we can say that
structural independence has been achieved. So, relational
database model has structural independence.
• Structural independence: In relational model, changes in the
database structure do not affect the data access. When it is
possible to make change to the database structure without
affecting the DBMS's capability to access data, we can say that
structural independence has been achieved. So, relational
database model has structural independence.
• Design, implementation, maintenance and usage ease: The
relational database model achieves both data independence and
structure independence making the database design,
maintenance, administration and usage much easier than the
other models.

ER Modeling and Introduction to RDBMS

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