Database standards (SQL92, ANSI, ISO, etc)
As always, the great thing about standards is that there are many to pick from. When it comes to SQL standards, there are different levels (Entry-Level, Intermediate, Advanced, etc) and different versions (SQL92, SQL 1999, SQL3, etc) to pick from. The databases above implement varying levels of these standards. What has also appears to have happened is that the ISO/ANSI standards are playing "catch up" with the database venders such as Oracle and PostgreSQL. Hence databases often have two syntaxes that can be used interchangably: there own existing one, plus the standards compliant one.
Typically, simple DML (Data Manipulation Language) queries using INSERT, SELECT, and UPDATE will work the same on each database. However, aggregate functions, join syntax, DDL (Data Definition Language) queries, stored procedures, and so forth often differ.
ODBC helps somewhat in this regard, since a vendor's ODBC driver will convert generic ODBC queries into their native query language.
Benchmarks
"The point is that Benchmarks Lie, particularly when they are getting trumpeted the most loudly." --cbbrowne
But, if you want more benchmarks, here are some links... add to the list if you find some good comparisons either between open source databases, or between open source and commercial ones.