App deployed on Heroku doesn't show data from MySql DB

Original Answer

Clever Cloud and Heroku are two different services. I don't think you can use the database from one in the other, nor do I understand why you would. Why not use Clever Cloud for both, keep your database and deploy your app to Clever Cloud? To deploy to Clever Cloud, take a look at their docs.

Now if you want to deploy your app to Heroku, your best bet is to migrate to postgres since that is the native database for Heroku. But you can use MySQL on Heroku if you wish. For that you can use an addon that Heroku provides, ClearDB MySQL addon from Heroku.

EDIT

Though I still think my advice is correct, my answer may NOT be. You should be able to use an outside database in your Heroku app, as long as that outside app accepts connections from Heroku. And it is easy to do as Shan Valleru's answer describes. Basically you just have to run

heroku config:add DATABASE_URL=mysql://user:[email protected]:3306/db-name
heroku config:add SHARED_DATABASE_URL=mysql://user:[email protected]:3306/db-name

Then do

heroku restart

OR

You can change these variables in the Heroku panel as d1jhoni1b describes in his answer:

1 - create mysql db
2 - create mysql db user (set defaults)
3.1 - Go to your Heroku panel/Config Vars
3.2 - Click on "Reveal Vars" and edit (clicking on pencil icon) on the one you want to change in this case DATABASE_URL (if not present just a new one with DATABASE_URL as the name)