Which SQL standard is most broadly and completely supported? I want to concentrate on one of the SQL language specs for learning and reference purposes. Which version of the SQL standard is most broadly supported across DBMS'es so that I can focus my time and energy on it ?
I think you are trying to live in Joe Celko's dream world where you can only use standard SQL and in any given week you may have to port all of your code from SQL Server to Oracle and then Oracle to DB2 and then back to SQL Server again. Twice.

While the core and fundamental aspects of standard SQL will help you anywhere, trying to limit yourself to that set of the language for fear of future porting (or just on principle) is not a path I'd recommend. Personally, I stick to ANSI standard stuff when I can (e.g. &lt;> vs. !=, COALESCE vs. ISNULL, CURRENT_TIMESTAMP vs. GETDATE(), etc.), but I'm also not afraid to use the SQL Server-specific stuff that makes my work easier.

It is important to understand how SQL works in general; it is equally important to understand how the language works in your RDBMS(es) - including deviations from the standard, deviations from the way another RDBMS might have implemented the same concept, and proprietary extensions that don't exist elsewhere.

SQL Server, as an example, covers a good portion of the standard, and it gets closer to full compliance with each new version. Will it ever cover 100%? Highly doubtful. Will it continue to add proprietary extensions not in the standard? Certainly. If everyone covered the standard and nobody stepped outside of it, then there would be no advantages to choosing one platform over another, and we'd all be using the same thing.