Divide monthly spend into daily spend in BigQuery - sql

Question

I have monthly data in BigQuery in the following form:
CREATE TABLE if not EXISTS spend (
id int,
created_at DATE,
value float
);
INSERT INTO spend VALUES
(1, '2020-01-01', 100),
(2, '2020-02-01', 200),
(3, '2020-03-01', 100),
(4, '2020-04-01', 100),
(5, '2020-05-01', 50);
I would like a query to translate it into daily data in the following day:
One row per day.
The value of each day should be the monthly value divided by the number of days of the month.
What's the simplest way of doing this in BigQuery?

Answer 1

You can make use of GENERATE_DATE_ARRAY() in order to get an array between the desired dates (in your case, between 2020-01-01 and 2020-05-31) and create a calendar table, and then divide the value of a given month among the days in the month :)
Try this and let me know if it worked:
with calendar_table as (
select
calendar_date
from
unnest(generate_date_array('2020-01-01', '2020-05-31', interval 1 day)) as calendar_date
),
final as (
select
ct.calendar_date,
s.value,
s.value / extract(day from last_day(ct.calendar_date)) as daily_value
from
spend as s
cross join
calendar_table as ct
where
format_date('%Y-%m', date(ct.calendar_date)) = format_date('%Y-%m', date(s.created_at))
)
select * from final

Answer 2

My recommendation is to do this "locally". That is, run generate_date_array() for each row in the original table. This is much faster than a join across rows. BigQuery also makes this easy with the last_day() function:
select t.id, u.date,
t.value / extract(day from last_day(t.created_at))
from `table` t cross join
unnest(generate_date_array(t.created_at,
last_day(t.created_at, month)
)
) u(date);