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Cc: pgsql-sql@postgresql.org
From: Michael Glaesemann <grzm@myrealbox.com>
Subject: Re: Determining Rank
Date: Fri, 4 Feb 2005 14:02:20 +0900
To: Don Drake <dondrake@gmail.com>


On Feb 4, 2005, at 12:06, Don Drake wrote:

> I have a query that shows the top N count(*)'s.
>
> So it's basically:
>
> select some_val, count(*)
> from big_table
> group by some_val
> order by count(*)
> limit 50
>
> Now, I would like to have the rank included in the result set.  The
> first row would be 1, followed by 2, etc. all the way to 50.

There are a couple of different ways to go about this. One is just to 
append an extra column that's basically a line number, but I find it 
doesn't handle ties very elegantly. The following example uses a 
correlated subquery using HAVING to determine the rank as "the number 
of items that have a total quantity greater than the current item + 1". 
Note that items bar and baz have exactly the same totals and are tied, 
while the rank of bat shows that there are 3 items that have totals 
greater than bat.

Joe Celko's "SQL for Smarties" has a bunch of things like this in it. 
I've found it quite helpful.

Hope this helps.

Michael Glaesemann
grzm myrealbox com

create table items (
     item text not null
     , qty integer not null
) without oids;

insert into items (item, qty) values ('foo', 1);
insert into items (item, qty) values ('foo', 2);
insert into items (item, qty) values ('foo', 1);
insert into items (item, qty) values ('foo', 3);
insert into items (item, qty) values ('foo', 3);
insert into items (item, qty) values ('foo', 20);
insert into items (item, qty) values ('foo', 1);
insert into items (item, qty) values ('bar', 3);
insert into items (item, qty) values ('bar', 1);
insert into items (item, qty) values ('bar', 3);
insert into items (item, qty) values ('bar', 13);
insert into items (item, qty) values ('baz', 2);
insert into items (item, qty) values ('baz', 4);
insert into items (item, qty) values ('baz', 14);
insert into items (item, qty) values ('bat', 3);
insert into items (item, qty) values ('bat', 4);

select item, sum(qty) as tot_qty
from items
group by item
order by tot_qty desc;

  item | tot_qty
------+---------
  foo  |      31
  bar  |      20
  baz  |      20
  bat  |       7
(4 rows)

select i1.item
     , i1.tot_qty
     , ( select count(*)
         from (
             select item
                 , sum(qty) as tot_qty
             from items
             group by item
             having sum(qty) > i1.tot_qty
             ) as i2
         ) + 1 as rank
from (
     select item
         , sum(qty) as tot_qty
     from items
     group by item
     ) as i1
order by i1.tot_qty desc;

  item | tot_qty | rank
------+---------+------
  foo  |      31 |    1
  bar  |      20 |    2
  baz  |      20 |    2
  bat  |       7 |    4
(4 rows)