CBSE Class 12 Computer Science: Introduction to SQL

Why SQL is the Secret Sauce of Modern Apps

Ever wondered how your favourite game stores high scores or how a shopping site remembers what you added to the cart? That's SQL at work, talking to a database behind the scenes.

💡 In Simple Words: SQL (Structured Query Language) is like a friendly waiter who takes your order (the question) and brings back the exact dish (the data) from a huge kitchen (the database). You just tell the waiter what you want, and it does the rest.

What is SQL?

SQL stands for Structured Query Language. Think of it as a special language that lets you ask a database to show, add, change, or delete information. It’s not a programming language like Java; it’s more like a set of commands you give to a librarian.

Key SQL Commands Every Class 12 Student Should Know

Here are the four big verbs you’ll use most often:

SELECT – asks the database to fetch data, like “show me all students who scored above 90”.
INSERT – adds a new record, like “add a new book to the library”.
UPDATE – changes existing data, like “change the price of a product”.
DELETE – removes data, like “throw away the broken toys”.

Writing Your First SELECT Query

Imagine a table called students with columns roll_no, name, and marks. To see every student’s name and marks, you write:

SELECT name, marks FROM students;

That line tells the database: “Give me the name and marks columns from the students table.”

Adding Data with INSERT

If you want to add a new student, you’d type:

INSERT INTO students (roll_no, name, marks) VALUES (101, 'Aisha', 88);

Think of it as putting a new card into a deck.

Changing Data with UPDATE

Suppose Aisha improves her marks to 92. You’d run:

UPDATE students SET marks = 92 WHERE roll_no = 101;

The WHERE part is crucial; without it, you’d change every row.

Removing Data with DELETE

To delete the record you just added:

DELETE FROM students WHERE roll_no = 101;

SQL Query Building Steps (Flowchart)

graph TD\nA[Identify required data] --> B[Choose SELECT clause] --> C[Add FROM table] --> D[Apply WHERE conditions] --> E[Sort with ORDER BY] --> F[Run the query]

Common Data Types in SQL

Just like a spreadsheet has different column types, SQL tables need to know what kind of data each column holds.

INT – whole numbers (e.g., age, roll number).
VARCHAR(n) – short text up to n characters (e.g., names).
DATE – calendar dates.
FLOAT – numbers with decimals.

Quick Comparison: SELECT vs INSERT vs UPDATE vs DELETE

Command	Purpose	Typical Syntax
SELECT	Read data	SELECT col FROM table WHERE condition;
INSERT	Add new row	INSERT INTO table (col1, col2) VALUES (val1, val2);
UPDATE	Modify existing rows	UPDATE table SET col=val WHERE condition;
DELETE	Remove rows	DELETE FROM table WHERE condition;

Tips for Writing Error‑Free SQL in Exams

Always end statements with a semicolon (;). It tells the system “that’s the whole command”.
Capitalize keywords (SELECT, FROM, WHERE) – it makes your answer neat and easier to read.
Use single quotes for text values and no quotes for numbers.
Double‑check column names; a tiny typo will cause “column not found” errors.

📝 Likely Exam Questions

Write an SQL query to display the name and salary of employees earning more than 50,000, sorted by salary descending.
```
SELECT name, salary FROM employees WHERE salary > 50000 ORDER BY salary DESC;
```
Insert a new record into the table books with id=10, title='SQL Basics', and price=299.
```
INSERT INTO books (id, title, price) VALUES (10, 'SQL Basics', 299);
```
Explain the difference between VARCHAR and INT data types with examples.
Answer: VARCHAR stores character strings, like a name “Riya”. INT stores whole numbers, like a roll number 23.
Update the marks of the student whose roll_no is 15 to 78.
```
UPDATE students SET marks = 78 WHERE roll_no = 15;
```
What will happen if you omit the WHERE clause in an UPDATE statement?
Answer: The database will change the specified column for every row in the table, which is usually a disaster.

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