Fill in missing rows when aggregating over multiple fields in Postgres

Question

I am aggregating sales for a set of products per day using Postgres and need to know not just when sales do happen, but also when they do not for further processing.

Answer 1

select 
    date, 
    count(sale_id) as sales, 
    product
from
    sales_data
    right join (
        (
            select d::date as date
            from generate_series (
                (select min(date) from sales_data),
                (select max(date) from sales_data),
                '1 day'
            ) gs (d)
        ) gs
        cross join
        (select distinct product from sales_data) p
    ) cj using (product, date)
group by product, date
order by product, date