The problem is that you are trying to access the element before it exists. You need to wait for the page to be fully loaded. A possible approach is to use the onload handler:
window.onload = function () {
var e = document.getElementById("db_info");
e.innerHTML='Found you';
};
Most common JavaScript libraries provide a DOM-ready event, though. This is better, since window.onload waits for all images, too. You do not need that in most cases.
Another approach is to place the script tag right before your closing </body>-tag, since everything in front of it is loaded at the time of execution, then.