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

Answer 2

You could use:

WITH cte AS (
   SELECT date, s.product
   FROM  ... -- some way to generate date series
   CROSS JOIN (SELECT DISTINCT product FROM sales_data) s
)
SELECT 
    c.date,
    c.product,
    COUNT(sd.sale_id) AS sales
FROM cte c
LEFT JOIN sales_data sd
  ON c.date = sd.date AND c.product= sd.product
GROUP BY c.date, c.product
ORDER BY c.date, c.product;

First create Cartesian product of dates and products, then LEFT JOIN to actual data and do calculations.

---

Oracle has great feature for this scenarios called Partitioned Outer Joins:

SELECT times.time_id, product, quantity 
FROM inventory  PARTITION BY  (product) 
RIGHT OUTER JOIN times ON (times.time_id = inventory.time_id) 
WHERE times.time_id BETWEEN TO_DATE('01/04/01', 'DD/MM/YY') 
      AND TO_DATE('06/04/01', 'DD/MM/YY') 
ORDER BY  2,1;