Implementation Specific User Manuals - SQL Reference

Beginners are often advised to RTM (Read the Manual). This is daft advice, most of us don't have that kind of life expectancy.

Postgres User manual

MySQL Language Reference

SQL Server documentation

Oracle Documentation (You may use user:htuttle password:mAb6RmR6 or register free yourself)

Tutorials

SQL for Web Nerds focusses on Oracle - the chapter on trees is particularly recomended.

SQL Tutorial w/ Live Practice Database & Interpreter

A tutorial by Jim Hoffman

SQL Tutorial from Chuo-Han Lee aims to teach beginners the building blocks of SQL.