beginner 
MySQL 101: Everything You Need to Know
A comprehensive guide to MySQL and SQL -- from basic concepts and DDL/DML to advanced queries, data integrity, transactions, and performance optimization.
Why Haskell? A Gentle Introduction to Functional Programming
An accessible introduction to Haskell — the purely functional programming language that makes math nerds blush with joy. Covers pure functions, lazy evaluation, type systems, monads, and real-world applications.