You have to use AJAX GET requests to retrieve JSON text from your server.
You can use PHP (or some other server-side language) to retrieve JSON from the database.
Do a Google search for "Construct2 mysql database" to find tutorials.
We use rex_hash to parse JSON state data, but I think the native Construct Dictionary object might work too.
Other valuable search terms:
php api tutorial -rest -soap
php prepared statement mysql
http get api parameters values