SQL拉取查询的INNER JOIN列中存在的值

Question

我正在尝试存储内部连接两个表的列中的值。不确定我的逻辑是否正确。截至目前，在选择option的下拉列表中， value 会存储到会话变量中，该变量将返回 NULL 。这是我打印结果的查询：

$query = 
"
SELECT c.emp_id
     , c.client_contact_id
     , c.project_id
     , p.project_id
     , p.project_codename
  FROM tlive_project_clist c
  JOIN tlive_project p 
    ON p.project_id = c.project_id 
 ORDER 
    BY p.project_codename
 ";

echo <select name="Projects">;

$result = mysqli_query($con, $query) or die("Query error: " . mysqli_error($con));
while ($row = mysqli_fetch_array($result)){  
  echo '<option value="'.$row['project_id'].'">'.$row['project_codename'].'</option>';
}

echo </select>;

php验证：

<?php
session_start();
require_once('php/config.php');

$project_id = $_POST['Projects'];

if(empty($project_id)){
  echo '*Please select a project';
}
else{               
$_SESSION['PROJECT_ID'] = $project_id;  
}
?>