Use results from one sql query in another where statement (subquery?)

Question

I see many similar questions but they're either so complex I can't understand them, or they don't seem to be asking the same thing.

It's simple: I have two columns: users (dmid) and downloads (dfid).

1. Select all users who downloaded a specific file:

   SELECT DISTINCT dmid FROM downloads_downloads where dfid = "7024"
   

2. Using the users above, find all the files they all downloaded:

   SELECT dfid from downloads_downloads WHERE dmid = {user ids from #1 above}
   

3. Count and order the dfid results , so we can see how many downloads each file received:

   dfid    dl_count_field
   ----    --------------
   18       103
   3        77
   903      66
   

My attempt at answering.

This seems close, but MySql bogs down and doesn't respond even after 30 seconds--I restart Apache eventually. And I do not now how to structure the count and order by without getting syntax errors because of the complex statement--and it may not even be the right statement.

SELECT dfid from downloads_downloads WHERE dmid IN (
    SELECT DISTINCT dmid FROM `downloads_downloads` where dfid = "7024")

Answer 1

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

or using a self join

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

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