online references.

Database:

• Official documentation
  • MariaDB | MySQL
  • PostgreSQL
  • MS SQL Server
  • Firebird
    • Firebird 4: Language Reference (PDF)
    • Firebird 3: Quick Start Guide | Language Reference | Developer’s Guide
    • Firebird 2.5: Quick Start Guide | Language Reference
  • SQLite
  • Advantage Database Server 12
• Tutoriais
  • SQLite => tutorialspoint (PDF, online) | sqlitetutorial | w3resource | techonthenet | Wikipedia
  • MariaDB => tutorialspoint (PDF, online) | techonthenet | pt-BR - MariaDB.com
  • MySQL => tutorialspoint | techonthenet | mysqltutorial
  • PostgreSQL => tutorialspoint | techonthenet | postgresqltutorial | PDF/Unicamp pgSQL v8 pt-br
  • Oracle => oracletutorial
  • SQL => Tutorial & References - w3schools.com
  • Connection Strings Reference - connectionstrings.com
• Docs/Books
  • MySQL Notes for Professionals book (PDF) - goalkicker.com