Dev

I made your insert and deletes into transactions, so they should roll back if one thing fails. 

I also stopped people from partially answering a poll, but there is a possibility of stashing incomplete answers.

To prevent null values in the database I corrected your IDs to have NOT NULL.

This will mean that will have to tell the few user that use this plugin to drop there tables becuase it seem unnecessary to put code in the plugin that removes the null values.