Question

I have a many-to-many table with approximately this structure:

id  | obj
----+---------
1   | 27
1   | 42
2   | 32
2   | 42
2   | 162
2   | 89
3   | 2
3   | 209

Essentially the table associates any number of objects (obj) with any number of collections (id).

I am trying to SELECT from this table in a way that will return the number of rows in a GROUP BY clause grouping by id, and also the number of grouped rows grouped by the number of rows in each group.

If I simply do SELECT COUNT(id) FROM table GROUP BY id, I naturally get the following:

id  | COUNT(id)
----+---------
1   | 2
2   | 4
3   | 2

That is, there is one row where COUNT(id) = 4 and there are two rows where COUNT(id) = 2. So far so good. But not what I’m looking for here.

What I need is this: for each distinct value returned by COUNT(id) (2 and 4 in this case), select both COUNT(id) and the number of rows that match that value in their COUNT(id) column (in this case 2 and 1, respectively: 2 rows have COUNT(id) = 2 and 1 row has COUNT(id) = 4).

In other words, from the table above, I would want this:

id_cnt | grp_cnt
-------+---------
2      | 2
4      | 1

– since grouping the table by id, you get two rows where COUNT(id) is 2 (ids 1 and 3); and one row where COUNT(id) is 4 (id 2).

Wrack my brain as I may, though, I cannot figure out a way to do this in one single query.

The closest I’ve been able to get to something that made sense in my head was this:

SELECT COUNT(*), id_cnt FROM table JOIN (SELECT COUNT(id) id_cnt FROM table GROUP BY id) a

– but that gives:

count(*) | id_cnt
---------+---------
21100    | 2

– which I admit confuses me a bit.

Can it be done?

(I find it odd that I can’t find this question already asked—surely this must have been asked before? Perhaps I’m just wording my search queries poorly…)

Answer 1

You can add another level of grouping around your first grouped query.

SELECT id_cnt, COUNT(*) AS grp_cnt 
FROM (
    SELECT COUNT(*) AS id_cnt 
    FROM test.test GROUP BY id) id_cnts
GROUP BY id_cnt;

Answer 2

You can get the cont by id this way

  select id, count(*) as ctn
  from table 
  group by id

and the count of same count with a second level count

 select ctn, count(*)
 from ( select id, count(*) as ctn
        from table 
        group by id) t
 group by ctn

(MySQL) Group by field and select both COUNT(field) and number of grouped rows

2 个答案: