Zero-filled daily sales grid for every store

A regional manager wants a complete store-by-day sales grid for the week of 20 to 24 October 2026. Days on which a store sold nothing must still appear, with sales of 0.

Using stores, report_days and daily_sales, return store_name, report_date and sales_amount (total sales for that store and day, 0 if none). Order by store_name, then report_date.

1. Step 1: `CROSS JOIN` builds every combination of store and day: 4 stores x 5 days = 20 rows.
2. Step 2: `LEFT JOIN` the real sales onto that grid so combinations with no sales are kept.
3. Step 3: Indiranagar has two rows on 20 October, so `SUM` per store and day is needed (35000).
4. Step 4: `COALESCE(SUM(...), 0)` turns the NULL produced for empty days into 0.

SELECT s.store_name,
       d.report_date,
       COALESCE(SUM(ds.sales_amount), 0) AS sales_amount
FROM stores s
CROSS JOIN report_days d
LEFT JOIN daily_sales ds
       ON ds.store_id  = s.store_id
      AND ds.sale_date = d.report_date
GROUP BY s.store_id, s.store_name, d.report_date
ORDER BY s.store_name, d.report_date;