Unfortunately, limiting yourself to only what is available in standard SQL is not practical. For example, doing idempotent inserts, insert-or-update, sequences, transactional behavior for DDL, data types, date/time manipulation, etc.

To paraphrase Tom Kyte from Oracle (Ask Tom): "Your company paid good money for Oracle and all of its features. Use them instead of wasting money reimplementing them poorly yourself."

There are many ways to achieve portability across databases, but limiting yourself to standard SQL is not the best way (at least for everything). And I say this as someone who develops an analytic database (Presto) that tries to follow standard SQL for everything.