koshikos

Может быть так?

<td><a href="add_emp_to_training.php?id=<?php echo $id;?>&training_id=<?php echo $get_id;?>" class="btn btn-green">Записать на тренинг</a></td>

<?php
include('dbcon.php');
include('session.php');

$id=$_GET['id'];
$get_id=$_GET['training_id'];

SELECT * FROM employee WHERE employeeID NOT IN (SELECT id_employee FROM employee_to_training WHERE id_training = < id >)