There is many, many database-related links on the Web. Here are a few worth investigating:

SQL links:
• mySQL manual (local copy)
• PostgreSQL: Introduction and Concepts by Bruce Momjian+
• Google's SQL web directory

Open source database management software:

• mySQL.org
• PostgreSQL

Programming web interfaces for databases:

• PHP
• PHP manual (local copy)
• Webmonkey's database pages

Miscellaneous:

• Database debunkings