h apter

C
8 Database Concepts

“Inconsistency of your mind… Can

damage your memory… Remove the
inconsistent data… And keep the original
one !!!”
— Nisarga Jain

In this Chapter
»» Introduction
»» File System
»» Database Management
System 8.1 Introduction
»» Rational Data Model After learning about importance of data in the
»» Keys in a Relational previous chapter, we need to explore the methods
Database to store and manage data electronically. Let us
take an example of a school that maintains data
about its students, along with their attendance
record and guardian details.
The class teacher marks daily attendance of the
students in the attendance register. The teacher
records ‘P’ for present or ‘A’ for absent against
each student’s roll number on each working day.
If class strength is 50 and total working days
in a month are 26, the teacher needs to record
50 × 26 records manually in the register every
month. As the volume of data increases, manual
data entry becomes tedious. Following are some
of the limitations of manual record keeping in
this example:

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 Activity 8.1 1) Entry of student details (Roll number and name)
in the new attendance register when the student is
Visit a few shops where
records are maintained promoted to the next class.
manually and identify 2) Writing student details on each month’s attendance
a few limitations of
page where inconsistency may happen due to
manual record
keeping faced by incorrectly written names, skipped student
them. records, etc.
3) Loss of data in case attendance register is lost
or damaged.
4) Erroneous calculation while consolidating
attendance record manually.
The office staff also manually maintain student details
viz. Roll Number, Name and Date of Birth with respective
guardian details viz. Guardian name, Contact Number
and Address. This is required for correspondence with
guardian regarding student attendance and result.
Finding information from a huge volume of papers
or deleting/modifying an entry is a difficult task in pen
and paper based approach. To overcome the hassles
faced in manual record keeping, it is desirable to store
attendance record and student details on separate data
files on a computerised system, so that office staff and
teachers can:
1) Simply copy the student details to the new
attendance file from the old attendance file when
students are promoted to next class.
2) Find any data about student or guardian.
3) Add more details to existing data whenever a new
student joins the school.
4) Modify stored data like details of student or guardian
whenever required.
5) Remove/delete data whenever a student leaves
the school.

8.2 File System

A file can be understood as a container to store data in
a computer. Files can be stored on the storage device
of a computer system. Contents of a file can be texts,
computer program code, comma separated values
(CSV), etc. Likewise, pictures, audios/videos, web pages
are also files.
Files stored on a computer can be accessed directly
and searched for desired data. But to access data of a

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 file through software, for example, to display monthly
attendance report on school website, one has to write
computer programs to access data from files.
Continuing the example of attendance at school,
we need to store data about students and attendance
in two separate files. Table 8.1 shows the contents of
STUDENT file which has six columns, as detailed below:
RollNumber – Roll number of the student
SName – Name of the student
SDateofBirth – Date of birth of the student
GName – Name of the guardian
GPhone – Phone number of the student guardian
GAddress – Address of the guardian of the student
Table 8.1 STUDENT file maintained by office staff
Roll SDateof
SName GName GPhone GAddress
Number Birth
1 Atharv Ahuja 2003-05-15 Amit Ahuja 5711492685 G-35, Ashok Vihar,
Delhi
2 Daizy Bhutia 2002-02-28 Baichung 7110047139 Flat no. 5, Darjeeling
Bhutia Appt., Shimla
3 Taleem Shah 2002-02-28 Himanshu Shah 9818184855 26/77, West Patel
Nagar, Ahmedabad
4 John Dsouza 2003-08-18 Danny Dsouza S -13, Ashok Village,
Daman
5 Ali Shah 2003-07-05 Himanshu Shah 9818184855 26/77, West Patel
Nagar, Ahmedabad

6 Manika P. 2002-03-10 Sujata P. 7802983674 HNO-13, B- block, Preet

Vihar, Madurai

Table 8.2 shows another file called ATTENDANCE

which has four columns, as detailed below:
AttendanceDate – Date for which attendance was marked
RollNumber – Roll number of the student
SName – Name of the student
AttendanceStatus – Marked as P (present) or A (absent)
Table 8.2 ATTENDANCE file maintained by class teacher
AttendanceDate RollNumber SName AttendanceStatus
2018-09-01 1 Atharv Ahuja P
2018-09-01 2 Daizy Bhutia P
2018-09-01 3 Taleem Shah A
2018-09-01 4 John Dsouza P
2018-09-01 5 Ali Shah A
2018-09-01 6 Manika P. P

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 2018-09-02 1 Atharv Ahuja P
2018-09-02 2 Daizy Bhutia P
2018-09-02 3 Taleem Shah A
2018-09-02 4 John Dsouza A
2018-09-02 5 Ali Shah P
2018-09-02 6 Manika P. P

8.2.1 Limitations of a File System

File system becomes difficult to handle when number of
files increases and volume of data also grows. Following
are some of the limitations of file system:
(A) Difficulty in Access
Files themselves do not provide any mechanism to
retrieve data. Data maintained in a file system are
accessed through application programs. While writing
such programs, the developer may not anticipate all
the possible ways in which data may be accessed. So,
sometimes it is difficult to access data in the required
format and one has to write application program to
access data.
(B) Data Redundancy
Redundancy means same data are duplicated in
different places (files). In our example, student names
are maintained in both the files. Besides, in Table
8.1, students with roll numbers 3 and 5 have same
guardian name and therefore same guardian name
is maintained twice. Both these are examples of
redundancy which is difficult to avoid in a file system.
Redundancy leads to excess storage use and may
cause data inconsistency also.
(C) Data Inconsistency
Data inconsistency occurs when same data maintained
in different places do not match. If a student wants to
get changed the spelling of her name, it needs to be
changed in SName column in both the files. Likewise, if
a student leaves school, the details need to be deleted
from both the files. As the files are being maintained by
different people, the changes may not happen in one of
the files. In that case, the student name will be different
(inconsistent) in both the files.
(D) Data Isolation
Both the files presented at Table 8.1 (STUDENT) and at
Table 8.2 (ATTENDANCE) are related to students. But

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 there is no link or mapping between them. The school Notes
will have to write separate programs to access these two
files. This is because data mapping is not supported in
file system. In a more complex system where data files
are generated by different person at different times, files
being created in isolation may be of different formats.
In such case, it is difficult to write new application
programs to retrieve data from different files maintained
at multiple places, as one has to understand the
underlying structure of each file as well.
(E) Data Dependence
Data are stored in a specific format or structure in a file.
If the structure or format itself is changed, all the existing
application programs accessing that file also need to
be changed. Otherwise, the programs may not work
correctly. This is data dependency. Hence, updating the
structure of a data file requires modification in all the
application programs accessing that file.
(F) Controlled Data Sharing
There can be different category of users like teacher,
office staff and parents. Ideally, not every user should
be able to access all the data. As an example, guardians
and office staff can only see the student attendance data
but should not be able to modify/delete it. It means
these users should be given limited access (read only)
to the ATTENDANCE file. Only the teacher should be
able to update the attendance data. It is very difficult to
enforce this kind of access control in a file system while
accessing files through application programs.

8.3 Database Management System

Limitations faced in file system can be overcome by
storing the data in a database where data are logically
related. We can organise related data in a database so
that it can be managed in an efficient and easy way.
A database management system (DBMS) or database
system in short, is a software that can be used to
create and manage databases. DBMS lets users to
create a database, store, manage, update/modify and
retrieve data from that database by users or application
programs. Some examples of open source and
commercial DBMS include MySQL, Oracle, PostgreSQL,
SQL Server, Microsoft Access, MongoDB.

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 A database system hides certain details about
how data are actually stored and maintained. Thus,
Some database it provides users with an abstract view of the data. A
management database system has a set of programs through which
systems include users or other programs can access, modify and retrieve
a graphical user the stored data.
interface for users to
create and manage The DBMS serves as an interface between the
databases. Other database and end users or application programs.
database systems Retrieving data from a database through special type of
use a command
commands is called querying the database. In addition,
line interface that
requires users to users can modify the structure of the database itself
use programming through a DBMS.
commands to Databases are widely used in various fields. Some
create and manage
applications are given in Table 8.3.
databases.
Table 8.3 Use of Database in Real-life Applications
Application Database to maintain data about
Banking customer information, account details, loan details,
transaction details, etc.
Crop Loan kisan credit card data, farmer’s personal data, land
area and cultivation data, loan history, repayment
data, etc.
Inventory product details, customer information, order details,
Management delivery data, etc.
Organisation employee records, salary details, department
Resource information, branch locations, etc.
Management
Online items description, user login details, users
Shopping preferences details, etc.

8.3.1 File System to DBMS

Let us revisit our school example where two data files
were maintained (Table 8.1 by office and Table 8.2 by
teacher). Let us now design a database to store data of
those two files. We know that tables in a database are
linked or related through one or more common columns
or fields. In our example, the STUDENT (Table 8.1) file
and ATTENDANCE (Table 8.2) file have RollNumber
and SName as common field names. In order to convert
these two files into a database, we need to incorporate
the following changes:
a) SName need not be maintained in ATTENDANCE
file as it is already there in STUDENT. Details for a
student can be retrieved through the common field
RollNumber in both the files.

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 b) If two siblings are in the same class, then same
guardian details (GName, GPhone and GAddress)
are maintained for both the siblings. We know this
is a redundancy and by using a database we can
avoid this. So let us split the STUDENT file into two High Cost is incurred
file (STUDENT file and GUARDIAN) file so that each while shifting from file
system to DBMS:
guardian data are maintained only once.
• Purchasing
c) One and more guardians can have the same name. sophisticated
So it will not be possible to identify which guardian hardware and
is related to which student. In such case, we need software.
to create an additional column, say GUID (Guardian • Training users for
ID) that will take unique value for each record in querying.
the GUARDIAN file. The column GUID will also be • Recurrent cost
kept with STUDENT file for relating these two files. to take regular
backup and perform
Note: We could distinguish guardians by their phone numbers recovery operations.
also. But, phone number can change, and therefore may not
truly distinguish guardian.
Figure 8.1 shows the related data files for the
STUDENT, GUARDIAN and ATTENDANCE details. Note
that this is not the complete database schema since it
does not show any relationship among tables.
STUDENT GUARDIAN ATTENDANCE
RollNumber GUID AttendanceDate
SName GName RollNumber
SDateofBirth GPhone AttendanceStatus
GUID GAddress

Figure 8.1: Record structure of three files in

STUDENTATTENDANCE database

The tables shown at Figure 8.1 are empty, which are

to be populated with actual data as shown in Table 8.4,
8.5 and 8.6.
Table 8.4 Snapshot of STUDENT table
RollNumber SName SDateofBirth GUID
1 Atharv Ahuja 2003-05-15 444444444444
2 Daizy Bhutia 2002-02-28 111111111111
3 Taleem Shah 2002-02-28
4 John Dsouza 2003-08-18 333333333333
5 Ali Shah 2003-07-05 101010101010
6 Manika P. 2002-03-10 466444444666

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 Table 8.5 Snapshot of GUARDIAN table
GUID GName GPhone GAddress
444444444444 Amit Ahuja 5711492685 G-35, Ashok Vihar, Delhi
111111111111 Baichung Bhutia 7110047139 Flat no. 5, Darjeeling Appt., Shimla
101010101010 Himanshu Shah 9818184855 26/77, West Patel Nagar, Ahmedabad
333333333333 Danny Dsouza S -13, Ashok Village, Daman
466444444666 Sujata P. 7802983674 HNO-13, B- block, Preet Vihar, Madurai

Table 8.6 Snapshot of ATTENDANCE table

Date RollNumber Status
2018-09-01 1 P
2018-09-01 2 P
2018-09-01 3 A
2018-09-01 4 P
2018-09-01 5 A
2018-09-01 6 P
2018-09-02 1 P
2018-09-02 2 P
2018-09-02 3 A
2018-09-02 4 A
2018-09-02 5 P
2018-09-02 6 P

Figure 8.2 shows a simplified database called

STUDENTATTENDANCE, which is used to maintain
data about the student, guardian and attendance. As
shown here, the DBMS maintains a single repository
of data at a centralised location and can be used by
multiple users (office staff, teacher) at the same time.

8.3.2 Key Concepts in DBMS

In order to efficiently manage data using a DBMS, let us
understand certain key terms:
(A) Database Schema
Database Schema is the design of a database. It is the
skeleton of the database that represents the structure
(table names and their fields/columns), the type of data
each column can hold, constraints on the data to be
stored (if any), and the relationships among the tables.
Database schema is also called the visual or logical
architecture as it tells us how the data are organised in
a database.

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 Teacher Office Staff

ry
Query Result

Qu

Query Result
e
Qu
er
y

DBMS Software processes Query

DBMS Software access database and its definition

Student
Database
Guardian Catalog

Attendance

Figure 8.2: STUDENTATTENDANCE database environment

(B) Data Constraint

Sometimes we put certain restrictions or limitations on
the type of data that can be inserted in one or more
columns of a table. This is done by specifying one or
more constraints on that column(s) while creating the
tables. For example, one can define the constraint that
the column mobile number can only have non-negative
integer values of exactly 10 digits. Since each student
shall have one unique roll number, we can put the NOT
NULL and UNIQUE constraints on the RollNumber
column. Constraints are used to ensure accuracy and
reliability of data in the database.
(C) Meta-data or Data Dictionary
The database schema along with various constraints on
the data is stored by DBMS in a database catalog or
dictionary, called meta-data. A meta-data is data about
the data.
(D) Database Instance
When we define database structure or schema, state
of database is empty i.e. no data entry is there. After
loading data, the state or snapshot of the database
at any given time is the database instance. We may
then retrieve data through queries or manipulate data

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 through updation, modification or deletion. Thus, the
state of database can change, and thus a database
schema can have many instances at different times.
(E) Query
A query is a request to a database for obtaining
information in a desired way. Query can be made to get
data from one table or from a combination of tables. For
example, “find names of all those students present on
Attendance Date 2000-01-02” is a query to the database.
To retrieve or manipulate data, the user needs to write
query using a query language called, which is discussed
in chapter 8.
Limitations of DBMS
(F) Data Manipulation
Increased Complexity:
Use of DBMS Modification of database consists of three operations
increases the viz. Insertion, Deletion or Update. Suppose, Rivaan joins
complexity of as a new student in the class then the student details
maintaining need to be added in STUDENT as well as in GUARDIAN
functionalities like files of the Student Attendance database. This is called
security, consistency,
Insertion operation on the database. In case a student
sharing and integrity
leaves the school, then his/her data as well as her
Increased data guardian details need to be removed from STUDENT,
vulnerability: GUARDIAN and ATTENDANCE files, respectively. This
As data are stored is called Deletion operation on the database. Suppose
centrally, it increases
the chances of loss
Atharv’s Guardian has changed his mobile number, his
of data due to any GPhone should be updated in GUARDIAN file. This is
failure of hardware or called Update operation on the database.
software. It can bring
(G) Database Engine
all operations to a halt
for all the users. Database engine is the underlying component or set of
programs used by a DBMS to create database and handle
various queries for data retrieval and manipulation.

8.4 Relational Data Model

Different types of DBMS are available and their
classification is done based on the underlying data model.
A data model describes the structure of the database,
including how data are defined and represented,
relationships among data, and the constraints. The most
commonly used data model is Relational Data Model.
Other types of data models include object-oriented data
model, entity-relationship data model, document model
and hierarchical data model. This book discusses the
DBMS based on relational data model.
In relational model, tables are called relations that
store data for different columns. Each table can have

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 multiple columns where each column name should be
unique. For example, each row in the table represents a
related set of values. Each row of Table 8.5 represents a
particular guardian and has related values viz. guardian’s
ID with guardian name, address and phone number.
Thus, a table consists of a collection of relationships.
It is important to note here that relations in a database
are not independent tables, but are associated with each
other. For example, relation ATTENDANCE has attribute
RollNumber which links it with corresponding student
record in relation STUDENT. Similarly, attribute GUID
is placed with STUDENT table for extracting guardian
details of a particular student. If linking attributes are
not there in appropriate relations, it will not be possible
to keep the database in correct state and retrieve valid
information from the database.
Figure 8.3 shows the relational database Student
Attendance along with the three relations (tables)
STUDENT, ATTENDANCE and GUARDIAN.

Figure 8.3: Representing STUDENTATTENDANCE database using Relational Data Model

Table 8.7 Relation schemas along with its description of Student Attendance
database
Relation Scheme Description of attributes
STUDENT(RollNumber, RollNumber: unique id of the student
SName, SDateofBirth, SName: name of the student
GUID) SDateofBirth: date of birth of the student
GUID: unique id of the guardian of the student
ATTENDANCE AttendanceDate: date on which attendance is taken
(AttendanceDate, RollNumber: roll number of the student
RollNumber, AttendanceStatus: whether present (P) or absent(A)
AttendanceStatus) Note that combination of AttendanceDate and RollNumber will be unique
in each record of the table
GUARDIAN(GUID, GUID: unique id of the guardian
GName, GPhone, GName: name of the guardian
GAddress) GPhone: contact number of the guardian
GAddress: contact address of the guardian

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 Each tuple (row) in a relation (table) corresponds
to data of a real world entity (for example, Student,
Guardian, and Attendance). In the GUARDIAN relation
(Table 8.5), each row represents the facts about the
guardian and each column name in the GUARDIAN table
is used to interpret the meaning of data stored under that
column. A database that is modeled on relational data
model concept is called Relational Database. Figure 8.4
shows relation GUARDIAN with some populated data.
Let us now understand the commonly used
terminologies in relational data model using Figure 8.4.
Relation GUARDIAN
with 4 attribute/
columns

GUID GName GPhone GAddress

444444444444 Amit Ahuja 5711492685 G-35, Ashok Vihar, Delhi
111111111111 Baichung Bhutia 7110047139 Flat no. 5, Darjeeling Appt., Shimla

Relation
State
101010101010 Himanshu Shah 9818184855 26/77, West Patel Nagar, Ahmedabad
333333333333 Danny Dsouza S -13, Ashok Village, Daman
466444444666 Sujata P. 7802983674 HNO-13, B- block, Preet Vihar, Madurai

Facts about RELATION GUARDIAN:

1. Degree (Number of attributes) = 4 Record/tuple/row
2. Cardinality (Number of rows/tuples/records) = 5
3. Relation is a flat file i.e, each column has a single value and each record
has same number of columns
Figure 8.4: Relation GUARDIAN with its attributes and tuples

i) ATTRIBUTE: Characteristic or parameters for

which data are to be stored in a relation. Simply
stated, the columns of a relation are the attributes
which are also referred as fields. For example, GUID,
GName, GPhone and GAddress are attributes of
relation GUARDIAN.
ii) TUPLE: Each row of data in a relation (table) is
called a tuple. In a table with n columns, a tuple is
a relationship between the n related values.
iii) DOMAIN: It is a set of values from which an attribute
can take a value in each row. Usually, a data type is
used to specify domain for an attribute. For example,
in STUDENT relation, the attribute RollNumber
takes integer values and hence its domain is a set of
integer values. Similarly, the set of character strings
constitutes the domain of the attribute SName.
iv) DEGREE: The number of attributes in a relation
is called the Degree of the relation. For example,
relation GUARDIAN with four attributes is a relation
of degree 4.

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 v) CARDINALITY: The number of tuples in a relation Notes
is called the Cardinality of the relation. For example,
the cardinality of relation GUARDIAN is 5 as there
are 5 tuples in the table.
8.4.1 Three Important Properties of a Relation
In relational data model, following three properties
are observed with respect to a relation which makes a
relation different from a data file or a simple table.
Property 1: imposes following rules on an attribute of
the relation.
• Each attribute in a relation has a unique name.
• Sequence of attributes in a relation is immaterial.
Property 2: governs following rules on a tuple of a
relation.
• Each tuple in a relation is distinct. For example, data
values in no two tuples of relation ATTENDANCE
can be identical for all the attributes. Thus, each
tuple of a relation must be uniquely identified by
its contents.
• Sequence of tuples in a relation is immaterial.
The tuples are not considered to be ordered, even
though they appear to be in tabular form.
Property 3: imposes following rules on the state of a
relation.
• All data values in an attribute must be from the
same domain (same data type).
• Each data value associated with an attribute
must be atomic (cannot be further divisible into
meaningful subparts). For example, GPhone of
relation GUARDIAN has ten digit numbers which
is indivisible.
• No attribute can have many data values in one
tuple. For example, Guardian cannot specify
multiple contact numbers under GPhone attribute.
• A special value “NULL” is used to represent
values that are unknown or non-applicable to
certain attributes. For example, if a guardian does
not share his or her contact number with the
school authorities, then GPhone is set to NULL
(data unknown).

8.5 Keys in a Relational Database

The tuples within a relation must be distinct. It means
no two tuples in a table should have same value for all
attributes. That is, there should be at least one attribute

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 Notes in which data are distinct (unique) and not NULL. That
way, we can uniquely distinguish each tuple of a relation.
So, relational data model imposes some restrictions or
constraints on the values of the attributes and how the
contents of one relation be referred through another
relation. These restrictions are specified at the time of
defining the database through different types of keys as
given below:
8.5.1 Candidate Key
A relation can have one or more attributes that takes
distinct values. Any of these attributes can be used
to uniquely identify the tuples in the relation. Such
attributes are called candidate keys as each of them
are candidates for the primary key.
As shown in Figure 8.4, the relation GUARDIAN
has four attributes out of which GUID and GPhone
always take unique values. No two guardians will have
same phone number or same GUID. Hence, these two
attributes are the candidate keys as they both are
candidates for primary key.
8.5.2 Primary Key
Out of one or more candidate keys, the attribute chosen
by the database designer to uniquely identify the tuples
in a relation is called the primary key of that relation.
The remaining attributes in the list of candidate keys
are called the alternate keys.
In the relation GUARDIAN, suppose GUID is
chosen as primary key, then GPhone will be called the
alternate key.
8.5.3 Composite Primary Key
If no single attribute in a relation is able to uniquely
distinguish the tuples, then more than one attribute
are taken together as primary key. Such primary key
consisting of more than one attribute is called Composite
Primary key.
In relation ATTENDANCE, Roll Number cannot be
used as primary key as roll number of same student
will appear in another row for a different date. Similarly,
in relation Attendance, AttendanceDate cannot be used
as primary key because same date is repeated for each
roll number. However combination of these two
attributes RollNumber and AttendanceDate together
would always have unique value in ATTENDANCE
table as on any working day, of a student would be

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 marked attendance only once. Hence {RollNumber,
AttendanceDate} will make the of ATTENDANCE relation
composite primary key.
8.5.4 Foreign Key
A foreign key is used to represent the relationship
between two relations. A foreign key is an attribute
whose value is derived from the primary key of another
relation. This means that any attribute of a relation
(referencing), which is used to refer contents from
another (referenced) relation, becomes foreign key if
it refers to the primary key of referenced relation. The
referencing relation is called Foreign Relation. In some
cases, foreign key can take NULL value if it is not the
part of primary key of the foreign table. The relation in
which the referenced primary key is defined is called
primary relation or master relation.
In Figure 8.5, two foreign keys in Student Attendance
database are shown using schema diagram where
the foreign key is displayed as a directed arc (arrow)
originating from it and ending at the corresponding
attribute of the primary key of the referenced table. The
underlined attributes make the primary key of that table.

STUDENT RollNumber SName SDateofBirth GUID

GUARDIAN GUID GName GPhone GAddress

ATTENDANCE AttendanceDate RollNumber AttendanceStatus

Figure 8.5: STUDENTATTENDANCE database with the primary and foreign keys

Summary
• A file in a file system is a container to store data in a computer.
• File system suffers from Data Redundancy, Data Inconsistency, Data
Isolation, Data Dependence and Controlled Data sharing.
• Database Management System (DBMS) is a software to create and manage
databases. A database is a collection of tables.
• Database schema is the design of a database.
• A database constraint is a restriction on the type of data that that can be
inserted into the table.

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 • Database schema and database constraints are stored in database catalog.
• The snapshot of the database at any given time is the database instance.
• A query is a request to a database for information retrieval and data
manipulation (insertion, deletion or update). It is written in Structured
Query Language (SQL).
• Relational DBMS (RDBMS) is used to store data in related tables. Rows
and columns of a table are called tuples and attributed respectively. A
table is referred to as a relation.
• Destructions on data stored in a RDBMS is applied by use of keys such as
Candidate Key, Primary Key, Composite Primary Key, Foreign Key.
• Primary key in a relation is used for unique identification of tuples.
• Foreign key is used to relate two tables or relations.
• Each column in a table represents a feature (attribute) of a record. Table
stores the information for an entity whereas a row represents a record.
• Each row in a table represents a record. A tuple is a collection of attribute
values that makes a record unique.
• A tuple is a unique entity whereas attribute values can be duplicate in
the table.
• SQL is the standard language for RDBMS systems like MySQL.

Exercise
1. Give the terms for each of the following:
a) Collection of logically related records.
b) DBMS creates a file that contains description about
the data stored in the database.
c) Attribute that can uniquely identify the tuples in
a relation.
d) Special value that is stored when actual data value is
unknown for an attribute.
e) An attribute which can uniquely identify tuples of the
table but is not defined as primary key of the table.
f) Software that is used to create, manipulate and
maintain a relational database.
2. Why foreign keys are allowed to have NULL values?
Explain with an example.
3. Differentiate between:
a) Database state and database schema
b) Primary key and foreign key
c) Degree and cardinality of a relation

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 4. Compared to a file system, how does a database Notes
management system avoid redundancy in data through
a database?
5. What are the limitations of file system that can be
overcome by a relational DBMS?
6. A school has a rule that each student must participate
in a sports activity. So each one should give only one
preference for sports activity. Suppose there are five
students in a class, each having a unique roll number.
The class representative has prepared a list of sports
preferences as shown below. Answer the following:
Table: Sports Preferences
Roll_no Preference
9 Cricket
13 Football
17 Badminton
17 Football
21 Hockey
24 NULL
NULL Kabaddi
a) Roll no 24 may not be interested in sports. Can
a NULL value be assigned to that student’s
preference field?
b) Roll no 17 has given two preferences sports. Which
property of relational DBMC is violated here? Can we
use any constraint or key in the relational DBMS to
check against such violation, if any?
c) Kabaddi was not chosen by any student. Is it possible
to have this tuple in the Sports Preferences relation?
7. In another class having 2 sections, the two respective
class representatives have prepared 2 separate Sports
Preferences tables, as shown below:
Sports preference of section 1 (arranged on roll number
column)
Table: Sports Preferences
Roll_no Sports
9 Cricket
13 Football
17 Badminton
21 Hockey
24 Cricket
Sports preference of section 2 (arranged on Sports name
column, and column order is also different)

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 Notes Table: Sports Preferences
Sports Roll_no
Badminton 17
Cricket 9
Cricket 24
Football 13
Hockey 21
Are the states of both the relations equivalent? Justify.
8. The school canteen wants to maintain records of items
available in the school canteen and generate bills when
students purchase any item from the canteen. The
school wants to create a canteen database to keep track
of items in the canteen and the items purchased by
students. Design a database by answering the following
questions:
a) To store each item name along with its price, what
relation should be used? Decide appropriate attribute
names along with their data type. Each item and its
price should be stored only once. What restriction
should be used while defining the relation?
b) In order to generate bill, we should know the quantity
of an item purchased. Should this information be in
a new relation or a part of the previous relation? If
a new relation is required, decide appropriate name
and data type for attributes. Also, identify appropriate
primary key and foreign key so that the following two
restrictions are satisfied:
i) The same bill cannot be generated for different
orders.
ii) Bill can be generated only for available items in
the canteen.
c) The school wants to find out how many calories
students intake when they order an item. In which
relation should the attribute ‘calories’ be stored?

9. An organisation wants to create a database EMP-

DEPENDENT to maintain following details about its
employees and their dependent.
EMPLOYEE(AadharNumber, Name, Address,
Department,EmployeeID)
DEPENDENT(EmployeeID, DependentName,
Relationship)
a) Name the attributes of EMPLOYEE, which can be
used as candidate keys.
b) The company wants to retrieve details of dependent
of a particular employee. Name the tables and the key
which are required to retrieve this detail.

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 c) What is the degree of EMPLOYEE and DEPENDENT
relation?
10. School uniform is available at M/s Sheetal Private
Limited. They have maintained SCHOOL_UNIFORM
Database with two relations viz. UNIFORM and COST.
The following figure shows database schema and its state.

School Uniform Database

Attributes and Constraints

Table: COST
UCode Size COST Price
Table: UNIFORM 1 M 500
Attribute UCode UName UColor 1 L 580
Constraints Primary Key Not Null - 1 XL 620
2 M 810
Table: COST 2 L 890
Attribute UCode Size Price 2 XL 940
Constraints Composite Primary Key >0
3 M 770
3 L 830
Table: UNIFORM 3 XL 910
UCode UName UColor 4 S 150
1 Shirt White 4 L 170
2 Pant Grey 5 S 180
3 Skirt Grey 5 L 210
4 Tie Blue 6 M 110
5 Socks Blue 6 L 140
6 Belt Blue 6 XL 160

a) Can they insert the following tuples to the UNIFORM

Relation? Give reasons in support of your answer.
i) 7, Handkerchief, NULL
ii) 4, Ribbon, Red
iii) 8, NULL, White
b) Can they insert the following tuples to the COST
Relation? Give reasons in support of your answer.
i) 7, S, 0
ii) 9, XL, 100
11. In a multiplex, movies are screened in different
auditoriums. One movie can be shown in more than one
auditorium. In order to maintain the record of movies,
the multiplex maintains a relational database consisting
of two relations viz. MOVIE and AUDI respectively as
shown below:
Movie(Movie_ID, MovieName, ReleaseDate)
Audi(AudiNo, Movie_ID, Seats, ScreenType,
TicketPrice)

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 a) Is it correct to assign Movie_ID as the primary
key in the MOVIE relation? If no, then suggest an
appropriate primary key.
b) Is it correct to assign AudiNo as the primary key in
the AUDI relation? If no, then suggest appropriate
primary key.
c) Is there any foreign key in any of these relations?

Student Project Database

Table: STUDENT
Roll No Name Class Section Registration_ID
11 Mohan XI 1 IP-101-15
12 Sohan XI 2 IP-104-15
21 John XII 1 CS-103-14
Table: PROJECT ASSIGNED
22 Meena XII 2 CS-101-14
Registration_ID ProjectNo
23 Juhi XII 2 CS-101-10
IP-101-15 101
Table: PROJECT IP-104-15 103
ProjectNo PName SubmissionDate CS-103-14 102
101 Airline Database 12/01/2018 CS-101-14 105
102 Library Database 12/01/2018 CS-101-10 104
103 Employee Database 15/01/2018
104 Student Database 12/01/2018
105 Inventory Database 15/01/2018
106 Railway Database 15/01/2018

12. For the above given database STUDENT-PROJECT,

answer the following:
a) Name primary key of each table.
b) Find foreign key(s) in table PROJECT-ASSIGNED.
c) Is there any alternate key in table STUDENT? Give
justification for your answer.
d) Can a user assign duplicate value to the field RollNo
of STUDENT table? Jusify.

13. For the above given database STUDENT-PROJECT, can

we perform the following operations?
a) Insert a student record with missing roll number
value.
b) Insert a student record with missing registration
number value.
c) Insert a project detail without submission-date.
d) Insert a record with registration ID IP-101-19 and
ProjectNo 206 in table PROJECT-ASSIGNED.

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