Thomas Harning's Reviews > SQL Pocket Guide: A Guide to SQL Usage
The complexity of developing database queries with SQL is a challenge often requiring frequent documentation searches. The "SQL Pocket Guide" by Jonathan Gennick is a great converged reference for many common database implementation.
The best feature of this guide is its breadth of detail offered. It provides a high-level view of database structures and provides useful details for taking strategies available in one implementation and possibly using it in another database engine. An example of this are the references from custom database function naming of Oracle's "analytic functions" and DB2's "OLAP functions" to the standard's name of "windowing functions". This allows you to take the naming you are familiar with, have been taught, or overheard and refer to it using that name and finding an appropriate redirection.
If you find that you are working with many different databases or want a quick reference to see if a given structure is available in a given database implementation, this guide is for you. Need a list of common data types for a category of data type: this guide has it. Need the details on dealing with times and dates: this guide has a good 20 pages on it. Even if it may not have all the tiny details you may need on a given topic, it can be a compass for finding your way through detailed documentation to what you want to find out.