How to extract email from “form_value” in wp_db7_forms table from database

Hi, I have installed this plugin to save contact form 7 data in database. but there is one issue, i need email value from this contact from 7 data. but this plugin saves contact form 7 data in one field named as “form_value”.

Please guide me in extracting the email value data from this table “wp_db7_forms” because i want that user will same email will register only one time so i need to restrict user on the basis of email.

Please guide me in resolving this issue. I will wait for your reply.

Thank you