Re: Determining Rank

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)