mysqli_query("SELECT DateDatetime,DateUpdate,Access,acn,opt,Date,Header,num FROM `dnevnik_zapisi` WHERE `visible`=1 AND `Access` IN ('all','podzamok','admin') AND (num IN (SELECT `num` FROM `dnevnik_tags` WHERE `tag`='пластилинки' AND `acn`=0)) AND `Access`='all' GROUP BY `num` ORDER BY DateDatetime DESC LIMIT 20")
'dnevnik.dnevnik_zapisi.DateDatetime' isn't in GROUP BY