At the moment, flexible_table::get_sql_sort can either be called as an instance method, or as a static method, but that last option is only used in one place in the assignment module.
Also, it does several things in one place, namely getting the list of sort columns, and then building the SQL from them. In the quiz report, we have some code that wants a list of columns, so it does get_sql_sort, and then explodes it. That is a bit silly. It would be better to have a separate method to get the list of columns as an array.
So, I am going to do a bit of refactoring to split this into separate methods. And change assignment, so the static method can be separate from the instance method.