units_main:
FACID----sub_key-----ASGMT_Name
AR01____1000_______LEGAL COMMAND
AR01____1001_______OPPS 67
AR02____1002_______REG D


units_sub:
FACID-----sub_key-----Long_Name
AR01_____1000_______cmd a, b and sometimes c
AR01_____1000_______secondary cmd structrue
AR01_____1001_______opps seg 798 b
AR02_____1002_______s cmd structure cc8

$real_facid = $_SESSION['facid'];

$query = "SELECT m.FACID
 , m.sub_key
 , m.ASGMT_Name
 , s.Long_Name
FROM units_main AS m
LEFT OUTER
JOIN units_sub AS s
ON s.FACID = m.FACID
AND s.sub_key = m.sub_key
WHERE m.FACID = '$real_facid'
ORDER
BY m.FACID
, m.sub_key
, s.Long_Name";


$result = mysql_query($query) or die(mysql_error());

$res = array();
while ($rows = mysql_fetch_array($result))
{ 
    if (!array_key_exists($res, $rows['ASGMT_Name']))
            {
                $res[$rows['ASGMT_Name']] = array();
            }
            $values = $res[$rows['ASGMT_Name']];
            array_push($values,$rows['Long_Name']);
}
foreach($res as $key => $value )
{
        echo "$key<br>";
        echo "$value<br>";
}

Here's the output:
LEGAL COMMAND
Array
OPPS 67
Array