SQLite returning groups with same number of rows in Android

Question

I have below SQLite query:

        select  
                product._id
                supermarket._id
                datetime(price.timestamp,'localtime') as LOCAL_TIMESTAMP,
                price.price
        from price inner join product on price.productid = product._id
                   inner join supermarket on price.supermarketid = supermarket._id

        order by price.productid, price.supermarketid, price.timestamp

Rows are ordered by productid, supermarketid and timestamp. Doing so they are grouped but not all pairs products-supermarket have the same timestamps so is it possible to obtain all pairs products-supermarket with the same number of timestamps? If a pair product-supermarket has not a timestamp, then set its price to 0 in the returned result.

For example, above query may return:

Product_A Supermarket_A "2014-03-10 00:00:00"   2.3
Product_A SUpermarket_A "2014-04-10 00:00:00"  15.0
Product_A SUpermarket_A "2014-04-20 00:00:00"  10.5

Product_B Supermarket_A "2014-01-01 00:00:00"  23.3
Product_B SUpermarket_A "2014-05-21 00:00:00"   1.0

and I would like to obtain:

Product_A Supermarket_A "2014-01-01 00:00:00"   0.0
Product_A Supermarket_A "2014-03-10 00:00:00"   2.3
Product_A SUpermarket_A "2014-04-10 00:00:00"  15.0
Product_A SUpermarket_A "2014-04-20 00:00:00"  10.5
Product_A SUpermarket_A "2014-05-21 00:00:00"   0.0

Product_B Supermarket_A "2014-01-01 00:00:00"  23.3
Product_B Supermarket_A "2014-03-10 00:00:00"   0.0
Product_B Supermarket_A "2014-04-10 00:00:00"   0.0
Product_B Supermarket_A "2014-04-20 00:00:00"   0.0
Product_B SUpermarket_A "2014-05-21 00:00:00"   1.0

In each product-supermarket pair appears all the timestamps (like an union). If a product-supermarket pair has not the timestamp, it is created and its price set to 0.0.

Is it possible to do in SQL?

Answer 1

To get all possible combinations of timestamps, join with the timestamps, but without a join condition. (The DISTINCTs are needed to avoid duplicates here.)

Then do an outer join with the prices:

SELECT productid,
       supermarketid,
       datetime(timestamp, 'localtime') AS local_timestamp,
       price.price
FROM (SELECT DISTINCT product._id     AS productid,
                      supermarket._id AS supermarketid
      FROM       product 
      INNER JOIN price       ON product._id         = price.productid
      INNER JOIN supermarket ON price.supermarketid = supermarket._id)
CROSS JOIN (SELECT DISTINCT timestamp
            FROM price)
LEFT JOIN price USING (productid, supermarketid, timestamp)

This returns NULL for missing prices. If you really want a zero, use IFNULL(price.price, 0.0) instead.