Revert all changes made in tables

Is there a way to revert a table to the same form as it was when I imported it from sql database file after I have modified it previously?

Yes, it's called "backup".

As soon as the database is updated you've literally overwritten old data. In some situations (depending on what you're doing) a typical Ctrl-Z may work for the last step, however if you alter the table structure and then decide to post the update your old state is lost. Except you have a backup, of course.

Thanks for the reply! With backup, does it mean I would duplicate the first table and use that as a backup?

Depending on what you are doing it would be better to backup the whole database. Just to be on the safe side.