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http://dev.mysql.com/doc/ The ultimate page to get the MySQL documentation. You can choose the language and the format file of the documentation (e.g.: Online HTML or pdf).
http://www.metatrontech.com/wpapers/mysql2postgresql.pdf Useful pdf for people who need to migrate from MySQL to PostreSQL. It gives the main differences between MySQL and PostgreSQL and the common problems that you have to face during the migration.