Class 12 Computer Science Chapter 8 NCERT Solutions – Database Concepts

Exercises

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 that is 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 of the table.
   6. Software that is used to create, manipulate and maintain a relational database.
   • Answer:
   • Table (Relation)
   • Data Dictionary (Meta-data)
   • Primary Key
   • NULL
   • Candidate Key / Alternate Key
   • Database Management System (DBMS)

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

   Answer: Foreign keys are allowed to have NULL values because a record might not always be associated with another table. For example, if a student doesn't have a guardian listed, the GUID (foreign key) in the STUDENT table can be NULL. This means the association is optional for that record.

3. Differentiate between:

   1. Database state and database schema
   2. Primary key and foreign key
   3. Degree and cardinality of a relation
   • Answer:
   • Database state is the data stored in a database at a particular moment; database schema is its design/structure (fields, types, relationships).
   • Primary key uniquely identifies each row in a table and cannot have NULLs; Foreign key is a field in one table referencing the primary key in another table; may have NULLs.
   • Degree is the number of attributes (columns) in a relation; cardinality is the number of tuples (rows).

4. Compared to a file system, how does a database management system avoid redundancy in data through a database?

   Answer: A DBMS stores data centrally in related tables, so repeated information is minimised. Instead of duplicating the same data in different files, linked tables with foreign keys minimise redundancy and ensure changes are reflected everywhere.

5. What are the limitations of file system that can be overcome by a relational DBMS?

   • Answer:
   • Redundancy and inconsistency
   • Data isolation
   • Difficulty in accessing data
   • Data dependence
   • Poor controlled data sharing

6. A school has a rule that each student must participate in a sports activity...

   • Roll no 24 may not be interested in sports. Can a NULL value be assigned to that student’s preference field?
   • Roll no 17 has given two preferences in sports...property violated? Can we use any constraint or key?
   • Kabaddi was not chosen by any student. Is it possible to have this tuple in the Sports Preferences relation?
   • Answer:
     • Yes, a NULL value can be assigned if the student hasn't selected any preference but the rule demands participation.
     • Giving two preferences violates the atomicity property. Use a UNIQUE or primary key constraint to prevent duplicate roll numbers or repeated preferences.
     • No, unless a student chooses Kabaddi, it should not exist as a tuple in the Sports Preferences relation. Unchosen sports are not entered in the relation.

7. In another class having 2 sections, Sports Preferences tables...

   • Are the states of both the relations equivalent? Justify.

   Answer: Yes, the states are equivalent because order of rows and columns has no impact in relations. If data (records) is the same, differences in arrangement do not matter.

8. The school canteen wants to maintain records ...

   • What relation should be used for items? Data types? Restrictions?
   • How to store quantity of items sold? New relation? Primary/foreign keys?
   • Where should the attribute ‘calories’ be stored?
   • Answer:
     • Create an Items table with ItemNo (Integer, Primary Key), ItemName (Varchar, not null), and Price (Float, not null).
     • Create a SaleOrders table with OrderNo (Integer, Primary Key), ItemNo (Foreign Key, not null), Quantity (Integer), Price (Float).
     • The calories attribute should be stored in the Items table as it is specific to each item.

9. An organisation wants to create a database EMP-DEPENDENT...

   • Name the attributes of EMPLOYEE which can be used as candidate keys.
   • Key/table needed to retrieve dependents' details for any employee?
   • What is the degree of EMPLOYEE and DEPENDENT relation?
   • Answer:
     • AadharNumber and EmployeeID can be candidate keys.
     • Use EmployeeID (foreign key) to link EMPLOYEE and DEPENDENT tables to get dependent details.
     • Degree of EMPLOYEE is number of attributes in EMPLOYEE; degree of DEPENDENT is number of its attributes (e.g., 5 and 3 if so designed).

10. School uniform is available at M/s Sheetal Private Limited...

    • Can they insert the following tuples to the UNIFORM Relation? Give reasons.
    • Can they insert the following tuples to the COST Relation? Give reasons.
    • Answer:
      • (7, Handkerchief, NULL): Can be inserted if UColor is not restricted as NOT NULL. (4, Ribbon, Red): Can be inserted if UCode is unique and all not-null constraints met. (8, NULL, White): Cannot be inserted if UName is NOT NULL.
      • (7, S, 0): Cannot be inserted because COST Price must be greater than 0. (9, XL, 100): Can be inserted if all constraints (positive price, valid UCode, etc.) are met.

11. In a multiplex, movies are screened in different auditoriums...

    • Is it correct to assign Movie_ID as the primary key in MOVIE? Alternatives?
    • Is it correct to assign AudiNo as the primary key in AUDI? Alternatives?
    • Is there any foreign key?
    • Answer:
      • Yes, Movie_ID can be the primary key in MOVIE if it is unique for each movie.
      • AudiNo alone is not sufficient if same auditorium screens different movies. Composite key—AudiNo+Movie_ID—is better.
      • Yes, the Movie_ID in AUDI is a foreign key referencing MOVIE's primary key.

12. For the above Student Project Database...

    • Name primary key of each table
    • Find foreign key(s) in table PROJECT-ASSIGNED.
    • Is there any alternate key in STUDENT?
    • Can a user assign duplicate value to RollNo of STUDENT?
    • Answer:
      • STUDENT: RollNo; PROJECT: ProjectNo; PROJECT-ASSIGNED: Registration_ID
      • ProjectNo in PROJECT-ASSIGNED is a foreign key referencing PROJECT table.
      • Yes, Registration_ID in STUDENT can serve as an alternate key as it is unique.
      • No, RollNo is a primary key and must be unique; duplicates are not allowed.

13. For the above Student Project Database, can we perform...

    • Insert a student record with missing roll number?
    • Insert with missing registration number?
    • Insert project detail without submission-date?
    • Insert record with registration ID IP-101-19 and ProjectNo 206 in PROJECT-ASSIGNED?
    • Answer:
      • No, RollNo is primary key and NOT NULL; missing value not allowed.
      • Yes, if Registration_ID allows NULLs as per schema.
      • Yes, if SubmissionDate is not marked NOT NULL.
      • No, cannot insert as ProjectNo 206 does not exist in PROJECT table—foreign key constraint violated.

NCERT Solutions Class 12 Computer Science Chapter 8 Database Concepts (2025-26) – Key Highlights

Mastering Database Concepts from this chapter helps you understand how data is organized, managed, and linked in modern computer systems. Grasping the role of keys and relational models will give you an extra edge in both board exams and future technology studies.

Regular practice of NCERT-based questions ensures you are ready for exam patterns and concepts like DBMS, file system limitations, and real-life database applications. Make notes while revising to remember key differences and definitions for quick last-minute review.

Stay focused on understanding primary, foreign, and composite keys for high-scoring answers. Use tables and real-world examples for clarity, and always check for conceptual clarity before moving to advanced database topics. Consistent revision will boost your confidence!