we should discuss how to refactor.

The DDL statements are a big mess, best illustrated here: https://manticore-projects.com/JSQLParser/syntax_snapshot.html#createtable
The syntax diagram does not even fit on the page.

I would like to advise for a refactoring which splits this one into many smaller, re-usable productions where grammar and code can be shared between CREATE and ALTER as much as possible -- based on the SQL:2016 standard.

A good blueprint maybe here: https://manticore-projects.com/SQL2016Parser/syntax.html#table-definition and https://manticore-projects.com/SQL2016Parser/syntax.html#alter-table-statement

Clean and neat as always, thanks a lot for working on this. Cheers!