Find the SQL-89 standard

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. <> 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.

I think it's an implementation detail.

A conforming implementation is not required to perform the exact sequence of actions defined in the General Rules, provided its effect on SQL-data and schemas, on host parameters and host variable, and on SQL parameters and SQL variables is identical to the effect of that sequence. The term effectively is used to emphasize actions whose effect might be achieved in other ways by an implementation.¹

I think an implementer could evaluate a common table expression 20 times, even in 20 different ways, and still have a conforming implementation. The only relevant issue is whether "its effect . . . is identical to the effect" of the sequence of actions defined in the General rules.

[1]. Section 6.3.3.3, "Rule evaluation order", in a draft of the SQL 2008 standard, having the local filename 5CD2-01-Framework-2006-01.pdf, p. 41 I have no idea where I got it. Google might know.