is it worth learning sql in sololearn rather than youtube channels?
Everyone has different learning style. SoloLearn SQL course is a good introduction, but the problem is it does not have a playground to practice. Same is true for YouTube, though. Most of the time, best way to learn these things is hands-on practice. So I recommend to install a database on your PC that you can practice with, or use some online services / playgrounds / coding platforms for the same, eg. http://sqlfiddle.com/
learn from a combination of the two. use sl to gain a basic understanding then watch yt vids and make notes for in depth understanding and specific use cases.