sql - Calculating Cumulative Sum in PostgreSQL

Question

I want to find the cumulative or running amount of field and insert it from staging to table. My staging structure is something like this:

ea_month    id       amount    ea_year    circle_id
April       92570    1000      2014        1
April       92571    3000      2014        2
April       92572    2000      2014        3
March       92573    3000      2014        1
March       92574    2500      2014        2
March       92575    3750      2014        3
February    92576    2000      2014        1
February    92577    2500      2014        2
February    92578    1450      2014        3          

I want my target table to look something like this:

ea_month    id       amount    ea_year    circle_id    cum_amt
February    92576    1000      2014        1           1000 
March       92573    3000      2014        1           4000
April       92570    2000      2014        1           6000
February    92577    3000      2014        2           3000
March       92574    2500      2014        2           5500
April       92571    3750      2014        2           9250
February    92578    2000      2014        3           2000
March       92575    2500      2014        3           4500
April       92572    1450      2014        3           5950

I am really very much confused with how to go about achieving this result. I want to achieve this result using PostgreSQL.

Can anyone suggest how to go about achieving this result-set?

Answer 1

Basically, you need a window function. That's a standard feature nowadays. In addition to genuine window functions, you can use any aggregate function as window function in Postgres by appending an OVER clause.

The special difficulty here is to get partitions and sort order right:

SELECT ea_month, id, amount, ea_year, circle_id
     , sum(amount) OVER (PARTITION BY circle_id
                         ORDER BY ea_year, ea_month) AS cum_amt
FROM   tbl
ORDER  BY circle_id, month;

And no GROUP BY.

The sum for each row is calculated from the first row in the partition to the current row - or quoting the manual to be precise:

The default framing option is RANGE UNBOUNDED PRECEDING, which is the same as RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW. With ORDER BY, this sets the frame to be all rows from the partition start up through the current row's last ORDER BY peer.

... which is the cumulative or running sum you are after. Bold emphasis mine.

Rows with the same (circle_id, ea_year, ea_month) are "peers" in this query. All of those show the same running sum with all peers added to the sum. But I assume your table is UNIQUE on (circle_id, ea_year, ea_month), then the sort order is deterministic and no row has peers.

Postgres 11 added tools to include / exclude peers with the new frame_exclusion options. See:

• Aggregating all values not in the same group

Now, ORDER BY ... ea_month won't work with strings for month names. Postgres would sort alphabetically according to the locale setting.

If you have actual date values stored in your table you can sort properly. If not, I suggest to replace ea_year and ea_month with a single column mon of type date in your table.

• Transform what you have with to_date():

    to_date(ea_year || ea_month , 'YYYYMonth') AS mon
  

• For display, you can get original strings with to_char():

    to_char(mon, 'Month') AS ea_month
    to_char(mon, 'YYYY') AS ea_year
  

While stuck with the unfortunate design, this will work:

SELECT ea_month, id, amount, ea_year, circle_id
     , sum(amount) OVER (PARTITION BY circle_id ORDER BY mon) AS cum_amt
FROM   (SELECT *, to_date(ea_year || ea_month, 'YYYYMonth') AS mon FROM tbl)
ORDER  BY circle_id, mon;