I also like https://learnxinyminutes.com for when I need to understand some code in an unfamiliar language. That site also accepts contributions from anyone via pull request on GitHub.
It's a pity that nobody has done one for SQL yet, I would've thought it would be in high demand.
You will often miss learning the philosophy of the language (the $language_name way of doing it), the different way of thinking about the problems that the language provides and similar details, which are often not necessary to read a snippet of code, but you want to understand it in order to properly learn the language and make the most of it.
SQL is a good example of it. It might be trivial to explain the syntax of most common SQL queries, but thinking in relations and tables is not natural to most people so there's not much you can do with basic syntax.