Re: grouping subsets

On Thu, Jul 22, 2010 at 11:31:23AM +0000, Tim Landscheidt wrote:
> Richard Huxton <dev(at)archonet(dot)com> wrote:
>
> >>> What I want to get is the values grouped by "subset", where a subset is a set of rows with identical column until the colum changes.
> >>> Is there a way to get
>
> >>> | 2 | B |
> >>> | 4 | C |
> >>> | 4 | B |
> >>> | 3 | D |
>
> >>> by SQL only?
>
> >> I think, the problem is that there are 2 identical groups. I think, you
> >> can write a pl/pgsql-proc, selecting all ordered by the date-field and
> >> walking through the result to do the grouping, checking if the 2nd
> >> column is different from the previous.
>
> >> With plain SQL it's maybe possible too, but i don't know how ...
>
> > It should be do-able in 8.4 onwards, look into windowing
> > functions. In particular the lag() function:
>
> > SELECT
> > mycode,
> > mydate,
> > lag(mycode) OVER (ORDER BY mydate) AS prev_code
> > FROM
> > mytable
> > ORDER BY mydate;
>
> > It should be possible to use that as a subquery with an
> > outer query that compares mycode=prev_code to get a run
> > length.
>
> Hmmm. Can the outer query be done without using "WITH
> RECURSIVE"?

How about this:

select
a, b, c, d, sum
from (
select
a, b, c, d, new_partition,
sum(e) over (partition by partition_num)
from (
select
a, b, c, d, e,
case when
lag(a, 1, null) over (order by d) is null or
lag(a, 1, null) over (order by d) != a or
lag(b, 1, null) over (order by d) is null or
lag(b, 1, null) over (order by d) != b or
lag(c, 1, null) over (order by d) is null or
lag(c, 1, null) over (order by d) != c
then nextval('a')
else currval('a')
end as partition_num,
case when
lag(a, 1, null) over (order by d) is null or
lag(a, 1, null) over (order by d) != a or
lag(b, 1, null) over (order by d) is null or
lag(b, 1, null) over (order by d) != b or
lag(c, 1, null) over (order by d) is null or
lag(c, 1, null) over (order by d) != c
then 'T'::boolean
else 'f'::boolean
end as new_partition
from foo
) bar
) baz
where
new_partition;

Here's my test table:
5432 josh(at)postgres# select * from foo;
a | b | c | d | e
---+---+---+------------------------+---
1 | 9 | 1 | 2007-01-01 00:00:05-07 | 8
9 | 2 | 1 | 2007-01-01 00:00:10-07 | 4
9 | 2 | 1 | 2007-01-01 00:00:15-07 | 6
9 | 2 | 1 | 2007-01-01 00:00:20-07 | 2
6 | 5 | 7 | 2007-01-01 00:00:25-07 | 3
4 | 9 | 0 | 2007-01-01 00:00:30-07 | 0
4 | 9 | 0 | 2007-01-01 00:00:35-07 | 7
5 | 2 | 7 | 2007-01-01 00:01:25-07 | 7
5 | 2 | 7 | 2007-01-01 00:01:30-07 | 7
5 | 2 | 7 | 2007-01-01 00:01:35-07 | 9
5 | 2 | 7 | 2007-01-01 00:01:40-07 | 2
5 | 2 | 7 | 2007-01-01 00:01:45-07 | 5
5 | 2 | 7 | 2007-01-01 00:01:50-07 | 8
5 | 2 | 7 | 2007-01-01 00:01:55-07 | 5
5 | 2 | 7 | 2007-01-01 00:02:00-07 | 9
7 | 8 | 8 | 2007-01-01 00:02:05-07 | 7
7 | 8 | 8 | 2007-01-01 00:02:10-07 | 8
9 | 3 | 0 | 2007-01-01 00:02:15-07 | 0
9 | 3 | 0 | 2007-01-01 00:02:20-07 | 8
9 | 2 | 1 | 2007-01-01 00:02:25-07 | 3
9 | 2 | 1 | 2007-01-01 00:02:30-07 | 3
(21 rows)

...and these results...
a | b | c | d | sum
---+---+---+------------------------+-----
1 | 9 | 1 | 2007-01-01 00:00:05-07 | 8
9 | 2 | 1 | 2007-01-01 00:00:10-07 | 12
6 | 5 | 7 | 2007-01-01 00:00:25-07 | 3
4 | 9 | 0 | 2007-01-01 00:00:30-07 | 7
5 | 2 | 7 | 2007-01-01 00:01:25-07 | 52
7 | 8 | 8 | 2007-01-01 00:02:05-07 | 15
9 | 3 | 0 | 2007-01-01 00:02:15-07 | 8
9 | 2 | 1 | 2007-01-01 00:02:25-07 | 6
(8 rows)

--
Joshua Tolley / eggyknap
End Point Corporation
http://www.endpoint.com