Asp.net MVC - call database on dropdown value change

MVC does not have a post-back mechanism as such as you had in WebForm (well, this was being carried out as javascript posting the form.

What you can do is create some javascript using jquery if you want to trigger some action when the item in that dropdown changes. it would be, (assuming the id of the dropdown is idDropDown

$('#idDropDown').change(function () {
    $.ajax(@Url.Action("AjaxAction", "MyController")', { selection : selectedValue }, function (data)){
        //handle ajax response here
    };
});

Your action controller will be like this:

public ActionResult AjaxAction(string selection)
{
    // do your server-side processing and get your data
    return Json(data);
}

With out jquery you can POST the data to controller action using pure java script.

@Html.DropDownListFor(m => m.SelectedValue,Model.SelectListItems,new{ onchange = "this.form.submit();" })

Note: Here simply you can submit form by adding java script code to on change. This is similar that you click on submit button.