Example of using `generate_series` in PostgreSQL to calculate a running sum for each customer

postgres_calc_running_sum_with_generate_series.sql

-- Run query here: https://bit.io/christoff12/explore_stuff?view=docs#/retail

with clean_data as (

    select
        customerid as customer_id,
        invoiceno as invoice_id,
        stockcode as stock_code,
        quantity,
        unitprice as unit_price,
        to_timestamp(invoicedate, 'MM/DD/YYYY HH24:MI') as invoiced_at,
        date_trunc('week', to_timestamp(invoicedate, 'MM/DD/YYYY HH24:MI'))::date as order_week

    from "retail"

),

step1 as (

    select
        customer_id,
        min(order_week) as min_order_week,
        max(order_week) as max_order_week
    
    from clean_data

    where customer_id is not null

    group by 1

),

step2 as (

    -- Creates a date spine for overall min, max range
    select 
        generate_series(date_range.min_date,
                        date_range.max_date, 
                        '1 week'::interval)::date as week
                        
    from (select 
            min(min_order_week) as min_date, 
            max(max_order_week) as max_date 
          from step1) as date_range

),

step3 as (

    -- Returns a row for every week in min, max range per customer_id
    select
        step1.*, 
        week

    from step1,
        lateral (select * from step2 where (week between min_order_week and max_order_week)) as week_fill

),

step4 as (

    select
        customer_id,
        order_week,
        coalesce(sum(case when invoice_id not like 'C%' then quantity end), 0) as pos_qty,
        coalesce(sum(case when invoice_id like 'C%' then quantity end), 0) as neg_qty

    from clean_data

    group by 1, 2

)

select
    step3.customer_id,
    step3.week,
    sum(pos_qty + neg_qty) over (partition by step3.customer_id 
                                 order by step3.week asc 
                                 rows between unbounded preceding and current row)

from step3
    left join step4 on step3.customer_id = step4.customer_id
        and step3.week = step4.order_week

where step3.customer_id = 16592 -- example (should return 42 rows)

order by step3.customer_id, step3.week