Ok I haven’t recommended a book in a while, and it’s not from the lack of reading them. I thought its time I recommend one of the books I always use on most of the projects I work on. SQL in a Nutshell from O'Reilly Press is my universal translator between database engines.
It’s the one book I take onsite with me, when I am working in multiple database environments. SQL In a Nutshell allows me to look at functions and syntax across database platforms. For instance ever wondered what CASE is in Oracle? Or what date functions are supported in MySQL? SQL In a Nutshell will give an overview of each SQL function and what is supported across database platforms.
This book also gives a great overview of the ANSI standard for SQL. The Second edition is well worth getting as it is updated with the SQL language definitions for Microsoft SQL Server, Oracle, MySQL, and PostgreSQL.
BTW: CASE in Oracle is DECODE go figure.