MySQL is a free database server, which usually use for webserver together with PHP or other database programing. The comment for query at MySQL is more less same with other SQL language, but some comment is only can be use at MySQL. Because there are so many think that we can use at MySQL, that's why usually we need a help when we want to explore more at MySQL.
If we have internet connection, that is easy to find what we want by using browser and typing the keyword at Google or other search engine, it must be find a lot of information about what we want to find, but as we know, maybe the result is not only for MySQL, the result might out anything according what are the keyword. We have to be careful to typing keyword for finding a better result, for example, “subquery +mysql” don't forget adding mysql as a keyword.
Maybe the problem become different when we don't have internet connection, because we can not use search engine to find the solutions. Ok, for this situation, we need a manual document help which is already prepare by MySQL community. The documentation manual are download able free for everyone and can be choose with some format type: PDF, HTML and CHM. Also there are some translations of the documentation at other languages than English.
Visit : http://dev.mysql.com/doc/
Yes, we need the internet connection for download this manual, but it only at the time we want to download a documentation manual, after we have it at our computer, than we can use it without internet connection. From that type of documentation format, I like using CHM (Windows help format), because it already index and have facility to find the function or anything.
Because the version of MySQL is always upgrade, it also recommended to find out the latest documentation. Visit the address http://dev.mysql.com/doc/ to watch the latest version. But of course we have to choose version of documentation manual same with the version of MySQL server.