SQL Basics: Learn Database Queries from Scratch

What is SQL?

SQL (Structured Query Language) is the standard language for managing and querying relational databases. It is used by databases like MySQL, PostgreSQL, SQLite, Oracle, and SQL Server. SQL skills are essential for data analysts, developers, and business intelligence professionals.

Basic SELECT Query

-- Select all columns
SELECT * FROM employees;

-- Select specific columns
SELECT name, salary, department FROM employees;

-- Filter with WHERE
SELECT * FROM employees WHERE salary > 50000;

-- Multiple conditions
SELECT * FROM employees
WHERE department = 'IT' AND salary > 40000;

-- Sort results
SELECT * FROM employees ORDER BY salary DESC;

-- Limit results
SELECT * FROM employees ORDER BY salary DESC LIMIT 10;

Aggregate Functions

-- Count records
SELECT COUNT(*) FROM employees;

-- Sum, Average, Min, Max
SELECT
  SUM(salary) as total_payroll,
  AVG(salary) as avg_salary,
  MIN(salary) as lowest,
  MAX(salary) as highest
FROM employees;

-- GROUP BY
SELECT department, COUNT(*) as headcount, AVG(salary) as avg_sal
FROM employees
GROUP BY department
HAVING AVG(salary) > 45000;

JOINs

-- INNER JOIN (only matching rows)
SELECT e.name, d.dept_name, e.salary
FROM employees e
INNER JOIN departments d ON e.dept_id = d.id;

-- LEFT JOIN (all from left table)
SELECT e.name, d.dept_name
FROM employees e
LEFT JOIN departments d ON e.dept_id = d.id;

INSERT, UPDATE, DELETE

-- Insert new record
INSERT INTO employees (name, salary, department)
VALUES ('Priya Sharma', 55000, 'Marketing');

-- Update record
UPDATE employees SET salary = 60000 WHERE id = 101;

-- Delete record
DELETE FROM employees WHERE id = 101;

Practice Platforms

Practice SQL for free on: SQLZoo, LeetCode (SQL section), HackerRank SQL, Mode Analytics, and W3Schools SQL playground.