ORACLE.
http://www.oracle.com/index.html

MySQL.
http://www.mysql.com/

SQL Tutorial.
http://www.1keydata.com/sql/sql.html

PL/SQL Programming Examples.
http://www.idevelopment.info/data/Programming/plsql/PROGRAMMING_PLSQL_Home.cgi

Oracle PL/SQL FAQ.
http://orafaq.com/faqplsql.htm

PL/SQL Tutorial - An introduction to using PLSQL with Oracle.
http://www.smart-soft.co.uk/Oracle/oracle-plsql-tutorial-part-11.htm

Developersdex SQL.
http://www.developersdex.com/sql/default.asp