Added SQL tutorial at w3schools (#4922)

free-programming-interactive-tutorials-en.md

@@ -282,6 +282,7 @@
 
 * [Intro to SQL: Querying and managing data](https://www.khanacademy.org/computing/computer-programming/sql) - Khan Academy
 * [SQL at Codecademy](https://www.codecademy.com/courses/learn-sql)
+* [SQL Tutorial](https://www.w3schools.com/sql) - W3Schools
 * [SQLBolt](http://sqlbolt.com)