我的数据库中的每个手段（来自表sv_orte）都是一个skiresort（表sv_ski）的成员 . 表sv_snowalrt表示每天的独特度假村 . 但是，GROUP BY查询在某些情况下无法加入skiresort . 是什么造成的？

SELECT b.res_id,b.resort,b.resort_us,SUM(alert_val) AS snowalert_val,SUM(alert_mid) AS snowalert_mid,SUM(alert) AS snowalert,SUM(precip) AS precip_sum,b.ski_id, a.lud
FROM sv_orte b
LEFT JOIN sv_snowalert a ON a.res_id = b.res_id 
LEFT JOIN sv_ski e ON b.ski_id = e.ski_id 
INNER JOIN sv_canton c ON c.can_id = b.can_id
INNER JOIN sv_country d ON d.cou_id = c.cou_id AND d.country='$country'
WHERE (a.lud BETWEEN '$today' AND DATE_ADD('$today', INTERVAL $day DAY) ) 
GROUP BY b.res_id HAVING snowalert>4 ORDER BY snowalert DESC LIMIT 100
mysql