Beginner
SQLZoo (sqlzoo.net)
w3schools (www.w3schools.com)
SQL Bolt (sqlbolt.com)
What Are Databases? (www.youtube.com)
Learn RDBMS in 6 Minutes (www.youtube.com)
What is Database & SQL? (www.youtube.com)
Defining Table Relationships (www.youtube.com)
Database Design (www.youtube.com)
Intro to SQL: Querying and Managing Data (www.khanacademy.org)
Difference Between Different Types of SQL (stackoverflow.com)
SQL vs NoSQL (www.thegeekstuff.com)
SQL Keys In Depth (begriffs.com)
Intermediate
Advanced