SELECT TIME FROM reception WHERE TIME NOT IN (SELECT TIME FROM doctor)

SELECT DISTINCT reception.time
FROM reception
LEFT JOIN doctor ON reception.time = doctor.time

$sql2 = "SELECT TIME FROM reception WHERE TIME NOT IN (SELECT TIME FROM doctor)";
$result_select2 = mysql_query($sql2);
echo "<select name = 'time'>";
while($row2 = mysql_fetch_array($result_select2)){
echo "<option value=' ".$row2['time']." '>".$row2['time']."</option>";
}
echo "</select>";
echo'<input type="submit">';
echo"</form>";

SELECT DISTINCT reception.time
FROM reception
LEFT JOIN doctor ON reception.time = doctor.time