如何在PHP中从数据库的时间戳中每隔6小时间隔计算一次数据组？

this is my query:

这是我的疑问：

$sql = "SELECT DATE( pm_timestamp ) AS d, HOUR( pm_timestamp ) AS h, 
       COUNT( * ) AS c FROM pm_impr_tracker WHERE campaign_id = '$camp_id' GROUP BY h,d order by `pm_timestamp`";
$sql = "SELECT DA