grouping every N values

aggregation, group-by, postgresql

Solution

The 1st approach that comes to mind is to use `row_number()` to annotate the table, then group by blocks of 16 rows.

SELECT min(id) as first_id, max(id) AS last_id, avg(rainfall) AS avg_this_16
FROM (
  SELECT id, rainfall, row_number() OVER (order by id) AS n
  FROM the_table
) x(id,rainfall,n)
GROUP BY n/16
ORDER BY n/16;

Note that this won't necessarily include 16 samples for the last group.

Alternately you can calculate a running average by using `avg()` as a window function:

SELECT id, avg(rainfall) OVER (ORDER BY id ROWS 15 PRECEDING)
FROM the_table;

... possibly annotating that with the row number and selecting the ones you want:

SELECT id AS greatest_id_in_group, avg_last_16_inclusive FROM (
  SELECT
    id, 
    avg(rainfall) OVER (ORDER BY id ROWS 15 PRECEDING) AS avg_last_16_inclusive,
    row_number() OVER (ORDER BY id) AS n
  FROM the_table
) x WHERE n % 16 = 0;

This will disregard the last n<16 samples, not returning a row for them.

Note that I'm assuming the IDs aren't guaranteed to be contiguous. If they are gap-less, you can just `group by id/16` and avoid the window function.

Problem

I have a table like this in PostgreSQL. I want to perform aggregation functions like mean and max for every 16 records based on ID (which is primary key). For example I have to calculate mean value for first 16 records and second 16 records and so on. ``` +-----+------------- | ID | rainfall | +-----+----------- | | 1 | 110.2 | | 2 | 56.6 | | 3 | 65.6 | | 4 | 75.9 | +-----+------------ ```

Original source