in the result query return set of select option like this

 while($row=$result->fetch_array()) {
    
    <select name="level[]" id="level[]">
      <option value=''>please select level</option>
      <option value='level 1'>Level one</option>
      <option value='level 2'>Level two</option>
      <option value='level 3'>Level three</option>
      <option value='level 4'>Level four</option>
    </select> 
    
    }
 <br><br>
    <input type="button" id="btn_level" value="choose level">
    

and jquery validation like this

$(document).ready(function() {

  $('#btn_level').click(function() {

    var level = $('#level[]').val();

    if (level == '') {
      alert('please select level');
    }

  });

});

above validation not work if use with array , but work for single select option, how to use jquery with array result ?

jsfiddle code playground