The data is stored in the wp_posts and wp_postmeta tables. You can see all the field values by doing this:
SELECT website from
(SELECT m1.meta_value as user_firstname, m2.meta_value as user_lastname, m3.meta_value as website
join dev_postmeta m1 on m1.post_id = ID and m1.meta_key = '_field_1'
join dev_postmeta m2 on m2.post_id = ID and m2.meta_key = '_field_2'
join dev_postmeta m3 on m3.post_id = ID and m3.meta_key = '_field_3'
where post_type = 'nf_sub') as v1
WHERE user_firstname = 'John' AND user_lastname = 'Allen'
The "x" in "_field_x" is the field ID in Ninja Forms.
Once you get the content, you can store it in any table.