[polygotdomain]

The short answer is to write ANSI 92 SQL and understand the variances between different RDBMS based on which ones you encounter.

The longer answer is that some RBDMS adhere closer to the standard than others. Generally the more open source and more long lived a platform is, the closer to the standard it can be. However, even an RDBMS like MSSqlServer isn't that far off from it, and while it may have things it supports that are outside of that standard, it will still support ANSI SQL (i.e. `ISNULL` vs `COALESCE`)

If you're looking for learning SQL that you can likely use in a wide number of places, I'd steer away from Oracle and DB2. Both are fairly proprietary, in my experience, and feel like writing in a different language that looks like SQL, but has a different set of rules and constraints.