2nd PUC Computer Science Chapter 8 Database Concepts Notes | NCERT Board 2026

This chapter explains the need for storing and managing data electronically. It begins with the limitations of manual record keeping and file systems, then introduces Database Management Systems (DBMS), the relational data model, important relational database terminology and different types of keys.

Data is important and needs to be stored and managed electronically. The chapter uses the example of a school that maintains student details, attendance records and guardian details.

In a manual attendance register, the teacher records P for present and A for absent against each student’s roll number for every working day.

Limitations of Manual Record Keeping

1. Student details have to be entered again in the new attendance register when a student is promoted to the next class.
2. Writing student details on every month’s attendance page may cause inconsistency because names may be written incorrectly or records may be skipped.
3. Data may be lost if the attendance register is lost or damaged.
4. Manual consolidation of attendance may result in erroneous calculations.

Why Computerised Data Storage?

Student and attendance details can be stored in separate data files on a computerised system. This allows office staff and teachers to:

1. Copy student details to the new attendance file when students are promoted.
2. Find information about a student or guardian.
3. Add details whenever a new student joins the school.
4. Modify stored student or guardian information when required.
5. Remove or delete data when a student leaves the school.

Student and Guardian Details

The office staff maintains student details such as Roll Number, Name and Date of Birth, along with guardian details such as Guardian Name, Contact Number and Address. These details are required for correspondence with guardians regarding attendance and results.

The contents of a file may include text, computer program code, comma separated values (CSV), pictures, audio/video and web pages.

Files stored on a computer can be accessed directly and searched for desired data. However, to access data from a file through software, application programs need to be written.

(A) Difficulty in Access

Files themselves do not provide a mechanism to retrieve data. Data in a file system is accessed through application programs. While developing such programs, all possible ways of accessing data may not be anticipated. Therefore, accessing data in the required format may become difficult and an application program may have to be written.

(B) Data Redundancy

In the school example, student names occur in both STUDENT and ATTENDANCE files. Guardian information may also be repeated. For example, students with roll numbers 3 and 5 have the same guardian name.

Redundancy results in excess storage usage and may also cause data inconsistency.

(C) Data Inconsistency

If a student’s name spelling is changed, it has to be changed in both files. Similarly, when a student leaves school, the details should be deleted from both files. Since different people may maintain different files, the changes may not happen everywhere, resulting in inconsistent data.

(D) Data Isolation

STUDENT and ATTENDANCE files contain related student information, but there is no link or mapping between them in a file system.

Separate programs have to be written to access the files. In a complex system, files created by different people at different times may have different formats. This makes it difficult to retrieve information from multiple files.

(E) Data Dependence

Data is stored in a specific format or structure in a file. If that structure or format changes, all existing application programs accessing that file also need to be changed.

(F) Controlled Data Sharing

Different users such as teachers, office staff and parents may need different levels of access. Not every user should be able to access all data.

For example, guardians and office staff may be allowed to view attendance but should not be able to modify or delete it. Only the teacher should be able to update attendance data.

Enforcing such access control through application programs in a file system is difficult.

The limitations of a file system can be overcome by storing data in a database where data is logically related.

DBMS allows users or application programs to create a database, store data, manage data, update or modify data and retrieve data.

In the school example, STUDENT and ATTENDANCE are separate files. Tables in a database can be linked or related through one or more common columns or fields.

STUDENT and ATTENDANCE have RollNumber and SName as common field names.

(a) Remove Repeated SName from ATTENDANCE

SName need not be maintained in ATTENDANCE because it is already available in STUDENT. Student details can be retrieved using the common field RollNumber in both files.

(b) Avoid Repeated Guardian Details

If two siblings are in the same class, the same guardian details (GName, GPhone and GAddress) may be maintained for both.

This is redundancy. To avoid it, the STUDENT file can be split into STUDENT and GUARDIAN files so that each guardian’s data is maintained only once.

(c) Create GUID

More than one guardian can have the same name. Therefore, guardian name alone cannot identify which guardian is related to which student.

An additional column called GUID (Guardian ID) is created. GUID takes a unique value for each record in the GUARDIAN file and is also stored in STUDENT to relate the two files.

Guardians could also be distinguished using phone numbers, but a phone number can change and therefore may not truly distinguish a guardian.

Cost of Shifting from File System to DBMS

• Purchasing sophisticated hardware and software.
• Training users for querying.
• Recurrent cost for regular backup and recovery operations.

DBMS and Centralised Data

The simplified STUDENTATTENDANCE database maintains a single repository of data at a centralised location and can be used by multiple users, such as office staff and teachers, at the same time.

(A) Database Schema

Database schema is also called the visual or logical architecture because it tells us how data is organised in a database.

(B) Data Constraint

For example, a mobile number column can be restricted to non-negative integer values of exactly 10 digits. Since each student should have one unique roll number, NOT NULL and UNIQUE constraints can be placed on the RollNumber column.

Constraints are used to ensure accuracy and reliability of data in a database.

(C) Meta-data or Data Dictionary

Meta-data means data about the data.

(D) Database Instance

When the database structure or schema is defined, the database state is empty because there is no data entry.

Data can then be retrieved through queries or manipulated through updation, modification or deletion. Since the database state can change, one database schema can have many instances at different times.

(E) Query

A query can retrieve data from one table or from a combination of tables.

Example given in the chapter: finding the names of all students present on a particular attendance date.

To retrieve or manipulate data, the user writes a query using a query language.

(F) Data Manipulation

Modification of a database consists of three operations:

(G) Database Engine

Limitations of DBMS Mentioned in the Chapter

Different DBMS types are classified based on their underlying data model. A data model describes the structure of a database, including how data is defined and represented, relationships among data and constraints.

The most commonly used data model is the Relational Data Model. Other data models mentioned in the chapter are:

• Object-oriented data model
• Entity-relationship data model
• Document model
• Hierarchical data model

Each table can have multiple columns, and each column name should be unique. Each row represents a related set of values.

For example, each row of the GUARDIAN relation represents a particular guardian with related values such as guardian ID, name, address and phone. Thus, a table consists of a collection of relationships.

Relations in a database are not independent tables; they are associated with each other.

ATTENDANCE has RollNumber which links it with the corresponding student record in STUDENT. GUID in STUDENT is used to obtain guardian details.

Attribute

Example: GUID, GName, GPhone and GAddress are attributes of the GUARDIAN relation.

Tuple

In a table with n columns, a tuple represents a relationship between the n related values.

Domain

Usually, a data type specifies the domain of an attribute. For example, RollNumber may take integer values and SName may take character strings.

Degree

Example: GUARDIAN has four attributes, so its degree is 4.

Cardinality

Example: GUARDIAN has 5 tuples, so its cardinality is 5.

Property 1 — Rules for Attributes

• Each attribute in a relation has a unique name.
• The sequence of attributes in a relation is immaterial.

Property 2 — Rules for Tuples

• Each tuple in a relation is distinct.
• No two tuples of ATTENDANCE can have identical values for all attributes.
• Each tuple must be uniquely identified by its contents.
• The sequence of tuples in a relation is immaterial.
• Tuples are not considered ordered even though they appear in tabular form.

Property 3 — Rules for the State of a Relation

• All data values in an attribute must belong to the same domain or data type.
• Each data value associated with an attribute must be atomic and cannot be further divided into meaningful subparts.
• An attribute cannot contain many data values in one tuple.
• NULL is used to represent unknown or non-applicable values.

Tuples within a relation must be distinct. No two tuples should have the same value for all attributes.

There should be at least one attribute whose value is unique and not NULL so that each tuple can be uniquely distinguished.

Restrictions on attribute values and relationships between relations are specified through different types of keys.

• The referencing relation contains the foreign key.
• The relation containing the referenced primary key is called the primary or master relation.
• A foreign key may take NULL if it is not part of the primary key of the foreign table.
• The foreign key is used to refer to information in another relation.

• A file in a file system is a container used to store data in a computer.
• File systems suffer from Data Redundancy, Data Inconsistency, Data Isolation, Data Dependence and Controlled Data Sharing limitations.
• DBMS is software used to create and manage databases.
• A database contains related tables.
• Database schema is the design of a database.
• A database constraint is a restriction on the type of data that can be inserted into a table.
• Database schema and constraints are stored in the database catalog.
• A database instance is the snapshot/state of a database at a particular time.
• A query is a request to a database for information retrieval and data manipulation.
• Data manipulation includes insertion, deletion and update.
• The relational DBMS stores data in related tables.
• Rows and columns of a table are called tuples and attributes respectively.
• A table is called a relation.
• Candidate Key, Primary Key, Composite Primary Key and Foreign Key are important keys.
• Primary key uniquely identifies tuples in a relation.
• Foreign key is used to relate two tables or relations.
• Each column represents an attribute.
• Each row represents a record or tuple.
• Degree is the number of attributes in a relation.
• Cardinality is the number of tuples in a relation.
• SQL is identified in the chapter as the standard language for RDBMS systems such as MySQL.

1 Mark

1. Define file.
2. What is data redundancy?
3. What is data inconsistency?
4. Define DBMS.
5. What is a database schema?
6. What is meta-data?
7. Define database instance.
8. What is a query?
9. Define attribute.
10. Define tuple.
11. What is domain?
12. Define degree and cardinality.
13. What is a candidate key?
14. What is a primary key?
15. What is a foreign key?

2 Marks

1. State the limitations of manual record keeping.
2. Explain data redundancy.
3. Explain data inconsistency.
4. Explain data dependence.
5. What are insertion, deletion and update?
6. Differentiate degree and cardinality.
7. What is a composite primary key?
8. Why are constraints used in a database?

3 Marks

1. Explain the limitations of a file system.
2. Explain candidate key and primary key.
3. Explain attribute, tuple and domain.
4. Explain degree and cardinality with examples.
5. Explain the three important properties of a relation.

4/5 Marks

1. Explain how a file system can be converted into a DBMS-based database.
2. Explain the important concepts in DBMS.
3. Explain the relational data model and its terminology.
4. Explain the different types of keys in a relational database.
5. Explain the three important properties of a relation.

1. Give the terms for each of the following

1. Collection of logically related records.
2. DBMS creates a file that contains description about the data stored in the database.
3. Attribute that can uniquely identify the tuples in a relation.
4. Special value stored when actual data value is unknown for an attribute.
5. An attribute which can uniquely identify tuples of the table but is not defined as primary key.
6. Software used to create, manipulate and maintain a relational database.

2.

Why are foreign keys allowed to have NULL values? Explain with an example.

3. Differentiate between

1. Database state and database schema
2. Primary key and foreign key
3. Degree and cardinality of a relation

4.

Compared to a file system, how does a database 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. Sports Preferences

1. Roll no 24 may not be interested in sports. Can a NULL value be assigned to that student’s preference field?
2. Roll no 17 has given two preferences in sports. Which property of relational DBMS is violated? Can a constraint or key be used to check against such violation?
3. Kabaddi was not chosen by any student. Is it possible to have this tuple in the Sports Preferences relation?

7. Sports Preferences of Two Sections

Section 1

Section 2

Are the states of both relations equivalent? Justify.

8. School Canteen Database

The school canteen wants to maintain records of items available and generate bills when students purchase items. The database should track items and items purchased by students.

1. To store each item name along with its price, what relation should be used? Decide appropriate attribute names and data types. Each item and its price should be stored only once. What restriction should be used?
2. To generate a bill, quantity of an item purchased is required. Should this information be in a new relation or part of the previous relation? If a new relation is required, decide its name and data types and identify primary and foreign keys satisfying the given restrictions.
   1. The same bill cannot be generated for different orders.
   2. A bill can be generated only for available items in the canteen.
3. The school wants to find how many calories students intake when they order an item. In which relation should the attribute calories be stored?

9. EMP-DEPENDENT Database

1. Name the attributes of EMPLOYEE which can be used as candidate keys.
2. The company wants to retrieve details of a dependent of a particular employee. Name the tables and key required to retrieve this detail.
3. What is the degree of EMPLOYEE and DEPENDENT relation?

10. School Uniform Database

Attributes and Constraints

UNIFORM

COST

1. Can the following tuples be inserted into UNIFORM? Give reasons:
   1. 7, Handkerchief, NULL
   2. 4, Ribbon, Red
   3. 8, NULL, White
2. Can the following tuples be inserted into COST? Give reasons:
   1. 7, S, 0
   2. 9, XL, 100

11. Multiplex Database

1. Is it correct to assign Movie_ID as the primary key in MOVIE? If no, suggest an appropriate primary key.
2. Is it correct to assign AudiNo as the primary key in AUDI? If no, suggest an appropriate primary key.
3. Is there any foreign key in any of these relations?

12. STUDENT-PROJECT Database

Answer the following:

1. Name the primary key of each table.
2. Find foreign key(s) in PROJECT-ASSIGNED.
3. Is there any alternate key in STUDENT? Give justification.
4. Can a user assign duplicate value to RollNo of STUDENT? Justify.

PROJECT

PROJECT-ASSIGNED

STUDENT

13. STUDENT-PROJECT Database Operations

Can the following operations be performed?

1. Insert a student record with missing roll number value.
2. Insert a student record with missing registration number value.
3. Insert a project detail without submission-date.
4. Insert a record with registration ID IP-101-19 and ProjectNo 206 in PROJECT-ASSIGNED.

Degree = Number of attributes in a relation

Cardinality = Number of tuples in a relation

GUARDIAN Degree = 4

GUARDIAN Cardinality = 5

Remember These Points

1. Manual record keeping becomes difficult as the volume of data increases.
2. A file is a container for storing data.
3. File systems have important limitations including redundancy, inconsistency, isolation and dependence.
4. DBMS is software for creating and managing databases.
5. DBMS provides an interface between users/application programs and databases.
6. Database schema represents the design and structure of a database.
7. Constraints restrict the type of data that can be inserted.
8. Meta-data is data about data.
9. Database instance is the state/snapshot of a database at a particular time.
10. Query is a request for information from a database.
11. Insertion, deletion and update are data manipulation operations.
12. Relational model represents data through related tables called relations.
13. Columns are attributes and rows are tuples.
14. Degree counts attributes; cardinality counts tuples.
15. Candidate keys can uniquely identify tuples.
16. One candidate key is selected as the primary key.
17. A candidate key not selected as primary key becomes an alternate key.
18. More than one attribute together can form a composite primary key.
19. A foreign key represents a relationship between relations.
20. NULL represents an unknown or non-applicable value.