SELECT dfid,count(*) 
from downloads_downloads 
WHERE dmid IN (
    SELECT dmid 
    FROM downloads_downloads 
    where dfid = "7024"
)
group by dfid

或使用自联接

select t1.dfid,count(*)
from downloads_downloads t1
inner join downloads_downloads t2
on t1.dmid = t2.dmid
where t2.dfid = "7024"

如果这花费的时间太长，那么您可能需要发布一个解释计划(谷歌它！)

if this takes too long then you will probably need to post an explain plan (google it!)