Question

I have a MySQL table with the following structure:

I want a query that would receive a group of uids (or a single uid) and then check for their existence in a closed group under a specific mid. If they exist, the query should return the mid under which they exist. For example in the table above:

('chuks.obima', 'crackhead') should return '2
('vweetah','crackhead') should return '1'
('vweetah','crackhead','chuks.obima') should return 3
('crackhead') should return an empty result

Answer 1

I think you need something like this:

SELECT mid
FROM your_table
WHERE uid in ('favour','crackhead','charisma')
GROUP BY mid
HAVING COUNT(*)=3

EDIT: based on your second example, this is what you are looking for:

SELECT mid
FROM your_table
WHERE uid in ('vweetah', 'crackhead')
GROUP BY mid
HAVING
  COUNT(distinct uid)=
  (select count(*)
   from (select 'vweetah' union select 'crackhead') s)

or you can just substitute last subquery with the number of elements you are looking for, e.g. HAVING COUNT(distinct uid) = 2

EDIT2: now i understand exactly what you are looking for. This should give you the correct results:

SELECT your_table.mid, s.tot_count, count(distinct uid)
FROM
  your_table inner join
  (select mid, seq, count(distinct uid) tot_count from your_table group by mid, seq) s
  on your_table.mid = s.mid and your_table.seq=s.seq
WHERE your_table.uid in ('crackhead')
GROUP BY your_table.mid
HAVING COUNT(distinct uid)=s.tot_count AND COUNT(distinct uid)=1

where the last count is equal to the number of elements you are looking for. This could be simplified like this:

SELECT your_table.mid
FROM your_table
GROUP BY your_table.mid
HAVING
  count(distinct uid)=
  count(distinct case when your_table.uid in ('vweetah','crackhead','chuks.obima') then your_table.uid end)
  and count(distinct uid)=3

If the group is to considered closed if all uid are under the same seq, you also have to modify group by with: group by your_table.mid, your_table.seq and your select with SELECT distinct your_table.mid