Обсуждение: Grouping, Aggregate, Min, Max

There is a trick to simplify the thing and avoid using aggregates :

I think it will give you your answer.

http://postgresql.1045698.n5.nabble.com/Count-of-records-in-a-row-td5775363i20.html

Cheers,

Rémi-C

2013/12/13 Misa Simic <misa.simic@gmail.com>

Hi All,

I am not sure how to define with words what I want to accomplish (so can't ask google the right question :) )

So will try to explain with sample data and expected result:

Scenario 1)

id	thing_id	category	period_id
1	1	A	1
2	1	A	2
3	1	A	3
4	1	A	4
5	1	A	5
6	1	A	6
7	1	A	7
8	1	A	8
9	1	A	9
10	2	A	1
11	2	A	2
12	2	A	3
13	2	A	4

Expected result:

thing_id	category	periods
1	A	1-9
2	A	1-4

(Sounds easy, group by, thing_id, category use Min and Max for period id - but further scenarios makes it a bit complicated...)

Scenario 2)

id	thing_id	category	period_id
1	1	A	1
2	1	A	2
3	1	A	3
4	1	A	4
5	1	B	5
6	1	B	6
7	1	B	7
8	1	A	8
9	1	A	9
10	2	A	1
11	2	A	2
12	2	A	3
13	2	A	4

Expected result:

thing_id	category	periods
1	A	1-4, 8-9
1	B	5-7
2	A	1-4

Scenario 3)

id	thing_id	category	period_id
1	1	A	1
2	1	A	2
3	1	A	3
4	1	A	7
5	1	A	8
6	1	A	9
7	2	A	1
8	2	A	2
9	2	A	3
10	2	A	4

Expected result:

thing_id	category	periods
1	A	1-3, 7-9
2	A	1-4

So goal is, to group by thing_id, category id - but if period_id is interupted (not in incremented by 1) to have aggregated spans...

To desired results we have came up using several CTE's (what makes a query a bit big, and more "procedural way": make cte what calculated diff between current and previous row, next cte uses previous one to define groupings, next cte to make aggregates etc...)

So I wonder - is there some kind of aggregate window function what does desired results?

Many Thanks,

Misa

Misa Simic <misa.simic@gmail.com> wrote:

> So I wonder - is there some kind of aggregate window function
> what does desired results?

Not built in, but PostgreSQL makes it pretty easy to do so.  With a
little effort to define your own aggregate function, your query can
look like this:

SELECT
    thing_id,
    category,
    int4range_list(period_id)
  FROM thing
  GROUP BY thing_id, category
  ORDER BY 1, 2;

I've attached a couple files -- one which creates the desired
aggregate function, and the other loads a table with two of your
sample data sets and runs the above.  This is just intended as a
quick example of the capabilities available to you.

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company