Class 12 Computer Science Chapter 9 Structured Query Language (SQL) – NCERT Solutions

1. Answer the following questions in about 30 words.

1. What is SQL? Name two popular RDBMS which use SQL as their query language.

Answer: SQL (Structured Query Language) is a standard programming language used to manage and manipulate relational databases. Two popular RDBMS that use SQL are MySQL and Oracle.

2. What is the difference between CHAR and VARCHAR data types in MySQL?

Answer: CHAR stores fixed-length character strings and always uses the same amount of space, even if not fully filled. VARCHAR stores variable-length strings and uses only as much space as needed for each value.

3. What is the use of PRIMARY KEY and FOREIGN KEY constraints in a table?

Answer: PRIMARY KEY uniquely identifies each row in a table and does not allow null values. FOREIGN KEY is used to establish a relationship between two tables by linking to the primary key of another table.

4. What is a composite key? Give an example from the chapter.

Answer: A composite key uses two or more columns together to uniquely identify a row in a table. In the ATTENDANCE table, AttendanceDate and RollNumber together form a composite primary key.

5. What are DDL and DML statements? Give one example of each.

Answer: DDL (Data Definition Language) statements are used to define or modify the database schema (e.g., CREATE TABLE). DML (Data Manipulation Language) statements are used to manipulate data (e.g., INSERT INTO).

2. Answer the following questions in about 150 words.

1. Explain the process and steps to create a relational database and associated tables in MySQL as discussed in the chapter.

Answer: To create a relational database, first install MySQL and start its service. Use the CREATE DATABASE statement to create a new database (e.g., CREATE DATABASE StudentAttendance;). Select the database using the USE statement. Then, create tables using CREATE TABLE, defining columns, datatypes, and constraints like PRIMARY KEY or FOREIGN KEY. For instance, the STUDENT table is created with columns RollNumber, SName, SDateofBirth, and GUID, specifying data types and primary key. Tables are linked using foreign keys to establish relationships, such as GUID in the STUDENT table referring to GUID in GUARDIAN. After table creation, data can be inserted using INSERT statements. Other optional steps include altering tables with the ALTER statement or removing them using DROP.

2. What is the importance of constraints in relational databases? Discuss at least three different types of constraints with examples from the chapter.

Answer: Constraints ensure data integrity and accuracy in relational databases. NOT NULL constraint ensures columns must be filled, e.g., SName in STUDENT is NOT NULL. UNIQUE constraint makes sure all values in a column are distinct, like GPhone in GUARDIAN. PRIMARY KEY uniquely identifies each row, as RollNumber in STUDENT, avoiding duplicate records. FOREIGN KEY creates relationships between tables, as GUID in STUDENT references GUARDIAN. DEFAULT sets a predefined value if none given; for example, SDateofBirth can have DEFAULT '2000-05-15'.

3. Match the following.

4. Write SQL statements for the following tasks based on the schema discussed in the chapter.

• (a) Create the GUARDIAN table (without constraints).
• (b) Add a primary key to the GUARDIAN table.
• (c) Add UNIQUE constraint to GPhone in GUARDIAN table.
• (d) Add a new column income of type INT to GUARDIAN table.
• (e) Remove the column income from GUARDIAN table.

Answer:

• (a) CREATE TABLE GUARDIAN(GUID CHAR(12), GName VARCHAR(20), GPhone CHAR(10), GAddress VARCHAR(30));
• (b) ALTER TABLE GUARDIAN ADD PRIMARY KEY (GUID);
• (c) ALTER TABLE GUARDIAN ADD UNIQUE(GPhone);
• (d) ALTER TABLE GUARDIAN ADD income INT;
• (e) ALTER TABLE GUARDIAN DROP income;

5. Differentiate between the following statements:

• (a) ALTER and UPDATE
• (b) DELETE and DROP

Answer:

• (a) ALTER is used to modify the structure of a table (e.g., add or remove columns, change data types), while UPDATE is used to change data values within table rows.
• (b) DELETE removes specific rows from a table based on a condition, while DROP deletes the entire table or database, including its structure and data.

Mastering Structured Query Language (SQL) in Class 12 Computer Science

Understanding SQL fundamentals is vital for scoring high in Computer Science Class 12. This NCERT Solutions chapter focuses on data definition, manipulation, and constraints in MySQL, helping you strengthen your basics and improve practical database skills.

By practicing chapter-wise NCERT exercises, students can learn core concepts such as DDL, DML, primary keys, foreign keys, and data types. Regular revision of these SQL queries ensures deeper understanding and fast problem-solving during exams.

Stay consistent with your preparation. Focus on solving exercise-based SQL questions, reviewing all constraints and commands. This approach not only boosts conceptual clarity but also builds confidence for practical computer science exams.