I came to MySQL when it looked like knowing PHP/MySQL would drastically increase my chances of a promotion from a server tech to a sysadmin. This was back in 2001 or so.
Really, I'm self taught with MySQL, Perl and PHP. I'm not the most knowledgeable, but learning by doing is key. I've found that the MySQL documentation online is great.
For examples, MySQL Cookbook is a good choice. It's oldish, however most of the syntax works with MySQL 5. http://is.gd/9DR