Partition by fixed number of records - sql

Question

I like to know how I can partition a window by a fixed number of records.
Example (http://sqlfiddle.com/#!1/7df86).
CREATE TABLE Games
(
id serial primary key,
game_no integer not null,
points integer,
constraint game_no unique (game_no)
);
INSERT INTO Games (game_no, points)
VALUES (3123, 5), (3126, 5), (3135, 8), (3128, null), (3130, 1), (3121, 11),
(3132, 0), (3133, 4), (3110, 7), (3112, null), (3113, 12), (3125, 3),(3134, 8);
I want the sum of the points of three games combined, starting with the highest game number, descending ordered by the game number. Like this.
| GAME_NO | POINTS | SUM_THREE |
|---------|--------|-----------|
| 3135 | 8 | 20 |
| 3134 | 8 | 20 |
| 3133 | 4 | 20 |
| 3132 | 0 | 1 |
| 3130 | 1 | 1 |
| 3128 | (null) | 1 |
| 3126 | 5 | 13 |
| 3125 | 3 | 13 |
| 3123 | 5 | 13 |
| 3121 | 11 | 23 |
| 3113 | 12 | 23 |
| 3112 | (null) | 23 |
| 3110 | 7 | 7 |
How to accomplish this with a window function without using a subquery? I also can't use for example a with statement. It has to be one single query because of the external parser that will execute it (and I have no control over). It seems so simple and I'm breaking my head over it the last couple of days:)

Answer 1

You can use row_number function divided by 3 to assign unique number to each group of 3 consecutive rows. Then use sum as analytical function on each group.
SQL Fiddle
with x(game_no, points, grp) as (
select game_no, points,
ceil(cast(row_number() over (order by game_no desc) as decimal)/ 3)
from games
)
select game_no, points,
sum(points) over (partition by grp)
from x
order by game_no desc;
You can use inline view instead of with construct.
select game_no, points,
sum(points) over (partition by grp)
from (
select game_no, points,
ceil(cast(row_number() over
(order by game_no desc) as decimal)/ 3) as grp
from games
) as x
order by game_no desc;
Results:
| GAME_NO | POINTS | SUM |
|---------|--------|-----|
| 3135 | 8 | 20 |
| 3134 | 8 | 20 |
| 3133 | 4 | 20 |
| 3132 | 0 | 1 |
| 3130 | 1 | 1 |
| 3128 | (null) | 1 |
| 3126 | 5 | 13 |
| 3125 | 3 | 13 |
| 3123 | 5 | 13 |
| 3121 | 11 | 23 |
| 3113 | 12 | 23 |
| 3112 | (null) | 23 |
| 3110 | 7 | 7 |