A great guide for learning SQL from the start, from Udemy:
https://blog.udemy.com/beginners-guide-to-sql/
What Wikipedia has to say about UDemy:
Udemy.com is a platform or marketplace for online learning. Unlike academic MOOC programs driven by traditional collegiate coursework, Udemy provides a platform for experts of any kind to create courses which can be offered to the public, either at no charge or for a tuition fee.[1]Udemy provides tools which enable users to create a course, promote it and earn money from student tuition charges.
In addition to SQL You will find there also the following tutorials:
