Since SQL is used by MySQL and other databases, you can find an endless supply of resources for this language. While generic SQL references will not necessarily show you how to get the most out of a MySQL database, they should remind you of the fundamentals. A few online SQL references include
Many bookstores stock texts dedicated to the language. You may find, however, that an entire book on SQL is more knowledge than all but the advanced database developer requires. Further, you'll see that MySQL does not support a number of features that would be discussed in an all-purpose SQL text. And, MySQL has its own extensions that you may not find in a general SQL book.