I don't know what exactly you're intending to do using this kind of client/server communication, but there are several things I'd like to point out:
*I see you're using a self written piece of code to send ajax requests (GET), instead I'd using a already given library, such as jQuery or MooTools, although i'd recommend jQuery, as the mootools project is discontinued.
Upside of mootools however is, that the code is understandable, many people don't understand the entire jQuery code.

*Creating a client-side query and the sending it to the server is a MAJOR security leak.
Unless you're going to use the program for private use, any user that could get past your code CAN and most likely WILL search the DOM for leaks.
This isn't a leak, its a hole in the wall, just sayin'.
Because this, basically, gives any user the privilege to execute MySQL query's without the need of your password/host/username and stuff.

*json_encode and json_decode are not supported on every server, if you're ever going to release your code, you might consider including the following code:
http://walotech.com/jsonsource.php

Below that code you see a part written by me, allowing support for any server, only defining the functions if they don't exist