I second this; languages and frameworks come and go and you definitely do not want to be left behind. What’s more important is learning how to learn, so you can pick up any language (“tool”) that comes your way.

> learn SQL or C.

And Lisp. Any Lisp. Clojure, Racket, Fennel, Janet, CL, Elisp.

What about Java?

Not Java. JVM. Many often confuse them. People dislike Java (not without merits), and because of that, they'd naturally dismiss JVM as a viable technological choice. JVM, as a matter of fact, is a pretty cool piece of technology. Learning the fundamentals of JVM and its internals is a good career investment without a doubt. And you don't really need to be a Java expert to build things to run on JVM.

Do you have any good link/books for learning about JVM?

I found that there are tons of beginner java, but nothing on JVM.

I've only found by Scott Oaks Java Performance, which gives some interesting knowledge about JVM but it's a secondary focus.

> learn SQL AND C.

FTFY. Both will last our lifetimes.