There's a book that every person who is serious about databases should read: it's "Applied Mathematics for Database Professionals", by APress.

See here: http://www.apress.com/9781590597453

Here is the table of contents, just to give you an idea of why it's such an amazing book:

Table of Contents

1. Logic: Introduction

2. Set Theory: Introduction

3. Some More Logic

4. Relations and Functions

5. Tables and Database States

6. Tuple, Table, and Database Predicates

7. Specifying Database Designs

8. Specifying State Transition Constraints

9. Data Retrieval

10. Data Manipulation

11. Implementing Database Designs in Oracle

12. Summary and Conclusions