Oracle is the most widely used platform and would be best to learn SQL for. Basically, you want to learn ANSI SQL, the SQL statements which are industry standard. Then each distribution (MS SQL, Oracle, MySQL, etc) all have their own extra functions which go beyond the ANSI standard.
If you want to learn SQL, the best resouce I've found is the Oracle OCA Exam Prep book for the SQL Fundamentals exam.
Learning SQL would be the best place to start.