Important Questions and Answers for Class 12 Computer Science Chapter 9 Structured Query Language (SQL) | 2026-27

Class 12 Computer Science Chapter 9 (MCQ)Multiple Choice Questions

Q1. Which SQL statement is used to retrieve data from a table?

(a) INSERT

(b) SELECT

(c) UPDATE

(d) DELETE

Answer: (b) SELECT

Explanation: The SELECT statement retrieves data from one or more tables. It can display all columns, selected columns or records that satisfy specified conditions.

Q2. Which of the following MySQL data types is used to store variable-length character strings?

(a) CHAR

(b) INT

(c) VARCHAR

(d) DATE

Answer: (c) VARCHAR

Explanation: VARCHAR(n) stores strings of variable length, up to the specified maximum. For example, VARCHAR(20) can store a student's name containing up to 20 characters.

Q3. Which constraint ensures that a column cannot contain NULL values?

(a) UNIQUE

(b) DEFAULT

(c) NOT NULL

(d) FOREIGN KEY

Answer: (c) NOT NULL

Explanation: The NOT NULL constraint prevents missing or unknown values from being stored in a column. For example, SName in the STUDENT table can be defined with NOT NULL.

Q4. Which SQL clause is used to arrange records in ascending or descending order?

(a) GROUP BY

(b) WHERE

(c) ORDER BY

(d) HAVING

Answer: (c) ORDER BY

Explanation: ORDER BY sorts query results according to specified columns. The default order is ascending, while DESC specifies descending order.

Q5. Which SQL function returns the total number of records in a table?

(a) SUM()

(b) COUNT(*)

(c) MAX()

(d) AVG()

Answer: (b) COUNT(*)

Explanation: COUNT(*) counts the rows in a table, including rows containing NULL values in individual columns.

Class 12 Computer Science Chapter 9 Very Short Answer Questions

Q6. What is Structured Query Language (SQL)? Mention its main uses.

Answer:

Structured Query Language (SQL) is a query language used to access and manage data in relational database management systems.

SQL is used to:

1. Create databases and tables.

2. Define attributes and constraints.

3. Insert, update and delete records.

4. Retrieve information using queries.

MySQL, Oracle and SQL Server are examples of database management systems that support SQL.

Q7. What is the difference between CHAR and VARCHAR data types?

Answer:

For example, storing 'city' in CHAR(10) involves padding the remaining six character positions with spaces. VARCHAR(30) stores the four-character value without requiring the full declared length.

Q8. What are SQL constraints? Name five commonly used constraints.

Answer:

SQL constraints are restrictions applied to the values stored in table columns to maintain data correctness.

Five commonly used constraints are:

1. NOT NULL: Prevents NULL values.

2. UNIQUE: Ensures distinct values in a column.

3. DEFAULT: Supplies a default value when none is provided.

4. PRIMARY KEY: Uniquely identifies each record.

5. FOREIGN KEY: Establishes a reference to the primary key of another table.

Q9. What is the purpose of the CREATE DATABASE and USE statements?

Answer:

The CREATE DATABASE statement creates a new database, while USE selects an existing database for performing operations.

Example:

CREATE DATABASE StudentAttendance;

USE StudentAttendance;

The first statement creates the StudentAttendance database. The second statement makes it the selected database.

Q10. What is the purpose of the DESCRIBE statement in MySQL?

Answer:

The DESCRIBE statement displays the structure of an existing table, including column names, data types, NULL permissions, keys and default values.

Example:

DESCRIBE STUDENT;

The statement displays the structure of the STUDENT table.

Q11. What is the difference between DELETE and DROP statements?

Answer:

Example:

DELETE FROM STUDENT

WHERE RollNumber = 3;

This removes the selected student's record.

DROP TABLE STUDENT;

This removes the entire STUDENT table.

Q12. What is the purpose of the DISTINCT keyword in SQL?

Answer:

DISTINCT removes duplicate values from the result of a SELECT query.

Example:

SELECT DISTINCT DeptId

FROM EMPLOYEE;

This displays each department ID only once, even if several employees belong to the same department.

Q13. What is the difference between WHERE and HAVING clauses?

Answer:

The WHERE clause filters individual records based on specified conditions.

The HAVING clause filters groups of records after applying GROUP BY.

Example using WHERE:

SELECT *

FROM EMPLOYEE

WHERE Salary > 5000;

Example using HAVING:

SELECT Paymentmode, COUNT(*)

FROM SALE

GROUP BY Paymentmode

HAVING COUNT(*) > 1;

The first query selects employees meeting a salary condition. The second selects payment-mode groups containing more than one sale.

Q14. What is the difference between single-row and aggregate functions?

Answer:

Single-row functions are also called scalar functions, while aggregate functions are called multiple-row functions.

Q15. What is a Cartesian product in a relational database?

Answer:

A Cartesian product combines every tuple of one relation with every tuple of another relation.

If the first relation contains m tuples and the second contains n tuples, the Cartesian product contains:

$m\times n$

tuples.

In SQL, a Cartesian product can be generated by including two tables in the FROM clause without a condition connecting them.

Class 12 Computer Science Chapter 9 Short Answer Questions and SQL Commands

Q16. Write SQL statements to create the STUDENT table with appropriate data types and a primary key.

Answer:

The STUDENT table contains RollNumber, SName, SDateofBirth and GUID.

Explanation:

• RollNumber stores integer values and is the primary key.

• SName stores names of up to 20 characters.

• SDateofBirth stores dates in YYYY-MM-DD format.

• GUID stores a fixed-length, 12-character guardian ID.

The primary key ensures that each student is uniquely identified by RollNumber.

Q17. Write SQL statements to add a primary key and a foreign key using ALTER TABLE.

Answer:

To make GUID the primary key of GUARDIAN:

ALTER TABLE GUARDIAN

ADD PRIMARY KEY (GUID);

To add GUID as a foreign key in STUDENT:

ALTER TABLE STUDENT

ADD FOREIGN KEY (GUID)

REFERENCES GUARDIAN(GUID);

Explanation: The first statement uniquely identifies each guardian. The second creates a relationship between STUDENT and GUARDIAN using GUID.

The referenced GUARDIAN table must already exist, and the linked columns must have compatible data types and sizes.

Q18. Write SQL statements to add, modify and remove an attribute from the GUARDIAN table.

Answer:

1. Add an Income attribute of type INT.

ALTER TABLE GUARDIAN

ADD Income INT;

2. Change GAddress from VARCHAR(30) to VARCHAR(40).

ALTER TABLE GUARDIAN

MODIFY GAddress VARCHAR(40);

3. Remove the Income attribute.

ALTER TABLE GUARDIAN

DROP Income;

Explanation: ALTER TABLE changes the structure of an existing table. It can add or remove columns and modify their data types or constraints without recreating the entire table.

Q19. Write SQL statements to insert the records of Atharv Ahuja and Taleem Shah into the STUDENT table.

Answer:

1. Insert Atharv Ahuja's details.

2. Insert Taleem Shah's details with NULL as GUID.

Explanation:

INSERT INTO is used to add records to a table.

Text and date values must be enclosed in single quotes. The GUID field for Taleem Shah contains NULL because the guardian ID is not provided.

The referenced guardian record for Atharv must already exist if the foreign key constraint is enforced.

Q20. Write an SQL query to retrieve the name and date of birth of the student whose roll number is 1.

Answer:

Output:

Explanation:

SELECT specifies the columns to retrieve, FROM identifies the table, and WHERE selects the record satisfying the condition RollNumber = 1.

Class 12 Computer Science Chapter 9 Long Answer Questions

For Questions 21–24, consider the EMPLOYEE table used in the OFFICE database.

21. Write an SQL query to display employee names and their annual income. Display the column headings as Name and Annual Income.

Answer:

Output:

Explanation:

The expression Salary * 12 calculates annual income using the monthly salary. AS provides a temporary name for the output column.

The calculated Annual Income column is displayed in the query result; it is not permanently added to the EMPLOYEE table.

Q22. Write an SQL query to display the names and department IDs of employees earning between ₹20,000 and ₹50,000.

Answer:

Output:

Explanation:

BETWEEN selects values within a specified range, including both boundary values.

The condition is equivalent to:

Q23. Write SQL queries to display employees in departments D01, D02 and D04, and to exclude employees belonging to D01 and D02.

Answer:

1. Display employees from D01, D02 and D04.

SELECT *

FROM EMPLOYEE

WHERE DeptId IN ('D01', 'D02', 'D04');

2. Display employees except those in D01 and D02.

SELECT *

FROM EMPLOYEE

WHERE DeptId NOT IN ('D01', 'D02');

Explanation:

IN checks whether a value matches any of the specified values. NOT IN excludes records matching those values.

The first query selects employees from three departments, while the second excludes employees of D01 and D02.

Q24. Write SQL queries to display employees in descending order of salary and to find employees whose bonus is NULL.

Answer:

1. Display employees in descending order of salary.

SELECT *

FROM EMPLOYEE

ORDER BY Salary DESC;

2. Display employees whose bonus is NULL.

SELECT *

FROM EMPLOYEE

WHERE Bonus IS NULL;

Output of the second query:

Explanation:

ORDER BY Salary DESC arranges employees from highest to lowest salary.

IS NULL is used to identify missing or unknown values. The condition Bonus = NULL should not be used to test for NULL.

Q25. Explain UPDATE and DELETE statements with SQL examples.

Answer:

UPDATE Statement

UPDATE modifies existing data in a table.

For example, to increase the salary of employee number 101 to ₹15,000:

UPDATE EMPLOYEE

SET Salary = 15000

WHERE EmpNo = 101;

DELETE Statement

DELETE removes records from a table.

For example, to delete the record of employee number 101:

DELETE FROM EMPLOYEE

WHERE EmpNo = 101;

Explanation:

• UPDATE changes values in existing records.

• DELETE removes the selected records.

• WHERE identifies the records affected by the operation.

If WHERE is omitted, UPDATE may modify all rows and DELETE may remove all rows from the table.

These commands should be executed independently when practising each operation.

Important Topics Covered in Class 12 Computer Science Chapter 9 Structured Query Language (SQL)

Chapter 9 explains how SQL is used to create, modify and retrieve information from relational databases.

The NCERT chapter includes practical examples of SQL commands and queries using database tables, making syntax and query writing important areas for exam preparation.

The following table highlights the major topics and concepts students should practise.

How Vedantu's Important Questions and Answers Help You Practise SQL in Chapter 9

Vedantu's Class 12 Computer Science Chapter 9 Important Questions and Answers help students understand SQL commands through solved queries, database examples and application-based practice.

• SQL Command-Based Questions: Practice CREATE, ALTER, INSERT, UPDATE, DELETE and SELECT statements with explanations of their uses.

• MCQs for Concept Revision: Test your understanding of SQL data types, constraints, functions, operators and different categories of SQL commands.

• Query-Writing Practice: Learn to write SQL statements for filtering, sorting, grouping and retrieving records from database tables.

• Function and Output-Based Questions: Understand how aggregate functions, single-row functions and SQL clauses affect query results.

• NCERT-Based Application Questions: Practise problems involving database tables, joins, record updates and retrieving information based on given conditions.

These important questions and solved answers help students recognise SQL syntax errors, understand the logic behind database queries and build confidence in answering both conceptual and practical questions.

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