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

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;