CODE EXAMPLE FOR SQL

check_username

$(document).ready(function(){

   $("#txt_username").keyup(function(){

      var username = $(this).val().trim();

      if(username != ''){

         $.ajax({
            url: 'ajaxfile.php',
            type: 'post',
            data: {username:username},
            success: function(response){

               // Show response
               $("#uname_response").html(response);

            }
         });
      }else{
         $("#uname_response").html("");
      }

   });

});