DropDown - Refresh list from MySQL

I have several DropDown on my project.They are populated by a MYSQL query. Often, my DropDown are empty when I open the page and if I do a refresh then the data is showed on the dropdown.

I am wondering if, like in AJAX, it is possible to refresh the dropdown only when the query is executed or any idea what I may doing wrong ?

Any idea will be welcome..

Thanks for your help