A few interesting links of database information I’ve run across:
Armstrong’s Axioms: Rules on functional dependencies.
Logic For Serious Database Folks: Chapters from a work in progress on Logic as it pertains to relational databases, no release date set for the complete work.
A Simple Guide to Five Normal Forms in Relational Database Theory: Covering the normal forms, most resources don’t go past Third Normal Form.
Comparison of different SQL implementations: Very comprehensive list of how several relational database products adhere or stray from the ANSI SQL standard.