SQL was standardized by ANSI and ISO. The most DBMS implement a majority these standards and add their proprietary extensions. Although the following list is not complete, it show the milestones of SQL standardization.

• SQL:2008 Legalizes ORDER BY outside cursor definitions. Adds INSTEAD OF triggers.
• SQL:2003 Introduced XML-related features, window functions, standardized
• SQL:1999 aka. SQL3 Added regular expression matching, recursive queries, triggers, support
  for procedural and control-of-flow statements, non-scalar types, and
  some object-oriented features.sequences, and columns with
  auto-generated values (including identity-columns).Adds the TRUNCATE
  statement.
• SQL-92 aka. SQL2 Major revision (ISO 9075)

So far so good. I currently found a very useful document of a DB-Admin from denmark, which compares diffent SQL implementations.

I reduced the information provided by Troels to the important facts, which are helpful for Reporting tasks on popular Data Warehouse DBMS (such as Oracle, IBM DB2 and MS SQL Server).

Checkout the attached pdf-document: SQL-Diffs.pdf