MySQL Reference Manual

This is the official MySQL Reference Manual. Separate editions of the book covering older versions of MySQL are also
available for download.

PostgreSQL: Introduction and Concepts

The book assumes no background in databases at all, but takes the reader from their first database query through to the complex queries needed to solve real-world problems. Highlights common pitfalls and offers many useful tips.

This online version of the book has been updated regularly and has additional chapters that will appear in the next print edition.

Practical PostgreSQL

This is a good reference to PostgreSQL. Besides explaining how to install and configure the database server, it
covers advanced features, such as versioning, replication, database maintenance, and backup & recovery,  needed in order to use PostgreSQL for serious business applications. There is an introduction to the PL/pgSQL programming language for developing stored procedures and triggers. Also includes a complete command reference.

This free online version includes a function to search inside the book.

SQL for Web Nerds

This is a free online sql book by Philip Greenspun, with a focus on using relational databases for building web services.

Topics include:

• data modeling
• simple and complex queries
• transactions
• triggers
• views,
• PL/SQL and Java
• trees
• Oracle-specific functionality
• data warehousing
• dealing with foreign and legacy data

Sams Teach Yourself Microsoft SQL Server 7 in 21 Days

A book in a tutorial format that aims to teach the skills you need to successfully install, administer, troubleshoot, and configure Microsoft SQL Server. Although designed more for administrators, many of the lessons apply to programmers as well.

Sams Teach Yourself SQL in 24 Hours

This book is aimed at readers with limited or no experience using SQL. It is a tutorial rather than a complete SQL reference, providing plenty of examples and exercises with which to apply the material covered.

Structured Query Language (SQL): a Practical Introduction

This is a good introductory text for learning SQL. It's concise and contains plenty of clear explanations and easy-to-follow examples.