Dropdown items conditional on sum of rows in different table

The code below works to create a dropdown list selection box with data from table COURSE.  Now I need to modify this so that only courses which have not reached their enrollment capacity are listed.  Table COURSE also has a field CAPACITY which is int and stores the number of seats in a course.  There is another table in my database which stores registration information, table STUDENTS which has the fields ID, USERID, COURSEID.  

I would like the dropdown list to be populated with COURSE.FULLNAMEs where the number of students has not reached COURSE.CAPACITY.  So I think I need something to calculate the number of times a particular COURSEID shows in table STUDENTS if that number is less than the relevant CAPACITY then go ahead and list the FULLNAME.  Please help with query language and php code.

-------------------------------------
<form action="registration_process.php" method="post" enctype="multipart/form-data">
     <tr>
            <td width="132" height="29" valign="top"><div align="left">Course Name:</div></td>
            <td width="423" valign="top"><input type="hidden" name="react" value="select_course">
                <select name="coursename" size="1">
<?php
$mysql_link = mysql_connect("localhost","user","pass");
mysql_select_db(moodle, $mysql_link);

$query = "SELECT fullname,id FROM course ";
$result = mysql_query($query, $mysql_link);
if(mysql_num_rows($result)) {
      // we have at least one course, so show all courses as options in select form
      while($row = mysql_fetch_row($result))
      {
            // the if is to exclude the site main page course from showing as a selection
            if ("1"!=$row[1]) print("<option value=\"$row[0]\">$row[0]</option>");
      }
} else {
      print("<option value=\"\">No courses created yet</option>");
}
?>
              </select></td>
            <td width="6" valign="top"><span class="style1">*</span></td>
          </tr>

--------------------