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SQL Introduction

What & Why

SQL (Structured Query Language) is the universal language for managing relational databases. It is used to store, retrieve, update, and delete data — and knowledge of SQL is required for virtually every software engineering role.

Syntax

sql

1-- SQL is NOT case sensitive, but convention capitalizes keywords
2SELECT column1, column2
3FROM   table_name
4WHERE  condition
5ORDER BY column1 ASC
6LIMIT 10;
7
8-- Comments
9-- Single line comment
10/* Multi-line
11   comment */

Real-World Example

Querying a product catalog database:

example.ts

sql

1-- Get top 10 bestselling electronics under ₹20,000
2SELECT
3    product_name,
4    category,
5    price,
6    units_sold,
7    (price * units_sold) AS total_revenue
8FROM products
9WHERE
10    category = 'Electronics'
11    AND price < 20000
12    AND is_active = TRUE
13ORDER BY units_sold DESC
14LIMIT 10;

Pitfalls & Interview Tips

⚠️SQL is declarative — you describe WHAT you want, not HOW to get it. The database optimizer decides the execution plan.
⚠️NULL is not a value — it means 'unknown'. NULL = NULL is false. Use IS NULL / IS NOT NULL.
💡Interview tip: The order of SQL clauses matters: SELECT → FROM → WHERE → GROUP BY → HAVING → ORDER BY → LIMIT.

1-- SQL is NOT case sensitive, but convention capitalizes keywords 2SELECT column1, column2 3FROM table_name 4WHERE condition 5ORDER BY column1 ASC 6LIMIT 10; 7 8-- Comments 9-- Single line comment 10/* Multi-line 11 comment */

1-- Get top 10 bestselling electronics under ₹20,000 2SELECT 3 product_name, 4 category, 5 price, 6 units_sold, 7 (price * units_sold) AS total_revenue 8FROM products 9WHERE 10 category = 'Electronics' 11 AND price < 20000 12 AND is_active = TRUE 13ORDER BY units_sold DESC 14LIMIT 10;