Of all the (programming) languages you can learn, I think SQL is the one that will help you the most throughout your career as a software developer. If you know the basics of SQL, you can solve set based problems directly in your database without writing a ton of code to do the same in an imperative way. And your applications will run much faster if they only fetch the data you really need.
Unfortunately, most tutorials for SQL are boring and all use the same approach to teach: You need to install a database server, restore the demo database and only then can you start running code to see what happens. Those obstacles can be enough to stop you from learning SQL and that is a shame.
SQLZoo.net is different. This free site allows you to poke around and run SQL commands right in the browser. No installation required; you take a few minutes out of your day and start learning. Topics range from basic SELECT statements to JOINS and aggregated functions. Everything is organised in a set of small exercises that take not much time. You start with a query that gives similar data back and it is now your task to modify it to get the right result. When you get it right, you get a visual feedback and you can jump into the next challenge:
I like the approach of SQL Zoo very much and I hope it helps you to learn SQL in a way that is interesting and fun. There is a big chance that SQL will be still relevant in 20 years and the sooner you learn SQL, the more you can profit.