Schaum's Outline of Fundamentals of Relational Databases

It is cheap so you can buy it in any case. But still I think it will not worth to buy.

This book is ideal for those of us who need to refresh knowledge and skills of relational databases that may have been unused for a few years, or for people who need to understand relational database concepts in a hurry. It starts with concepts that are essential to understanding relational databases, then introduces you to SQL. If you are working with Oracle you will find that standard SQL and Oracle's SQL*Plus are quite different in that SQL*Plus has a richer set of verbs. Regardless, the intro to SQL is valuable and it provides you with the ability to write queries for real world needs. The examples will work with DB2, Oracle of any other database that will understand standard SQL, making this book product-independent and invaluable.The sections that I especially liked covered normalization and entity-relationship models. These are basic knowledge factors that are essential to understanding relational databases, and are the heart and soul of the book.This book's value is that it's straightforward and presents a lot of material in a short number of pages. It is not for non-technical people in my opinion because the writing is sparse and terse. However, if you have an understanding of data or want a fast refresh this book will save you time by cutting through the fluff.