I don't mean "Basic SQL", but strongly the specs and the difference between the specs and the implementations between great databases (like SQL Server, Oracle, etc).
(source: amazon.com)
SQL In a Nutshell by O'Reilly and Associates. It covers all 5 major SQL Dialects, the differences between them, and does that for each function/feature listed. (ANSI SQL99, Oracle, MySql, Postgres, and SQL Server).