Introduction to SQL: SELECT Queries

What is SQL?

SQL (Structured Query Language) is how you talk to relational databases — asking for data, filtering it, and combining it across tables.

The Basic SELECT

SELECT name, email FROM users;

This returns the name and email columns for every row in the users table. Use * to select all columns, though naming columns explicitly is better practice.

Filtering with WHERE

SELECT * FROM users WHERE age >= 18 AND country = 'India';

Sorting and Limiting

SELECT title, views FROM tutorials ORDER BY views DESC LIMIT 10;

Joining Tables

Real data usually lives across multiple related tables. A JOIN combines rows from two tables based on a matching column:

SELECT orders.id, users.name FROM orders JOIN users ON orders.user_id = users.id;

Grouping and Aggregating

SELECT category, COUNT(*) AS total FROM tutorials GROUP BY category ORDER BY total DESC;

Common aggregate functions: COUNT(), SUM(), AVG(), MIN(), MAX().

A Common Beginner Mistake

Forgetting that WHERE filters rows before grouping, while HAVING filters groups after aggregation:

SELECT category, COUNT(*) AS total FROM tutorials GROUP BY category HAVING COUNT(*) > 5;

Practice Exercise

Given a students table and an enrollments table, write a query that lists each student’s name alongside how many courses they’re enrolled in, sorted highest first.