下拉框显示为空白而不是从数据库中提取项目

For some reason, my PHP code won't show up database in the drop down box that I retrieve from MySQL. CS_JOBS and CS_JOBS_CATEGORY are tables in the database. I join CS_JOBS_CATEGORY and CS_JOBS, since they both have the same category_code column. I already tested connect.php, it's working fine.

<?php 

   require("connect.php");

   $sql = "SELECT category_name FROM CS_JOBS_CATEGORY";
   $result = mysql_query($sql);
   echo "<select name='category_name'>";
   while ($row = mysql_fetch_array($result)) {
   echo "<option value='".$row['category_name']."'>".$row['category_name']."</option>";
   }
   echo "</select>";

?>

My goal is to show all category_name in the drop down box. Every time a user clicks on each category_name, it will show up all jobs(CS_JOBS) in each category. CS_JOBS consists of job_order_number, location, date_close ..etc..