MySQL group by where day <= x - mysql

Question

I need some help figuring out the correct SQL statement.
If've got a table with the following structure:
id, product_id, units, timestamp
I wan't a list which contains the all over units per day. A product has maximum one record per day.
So my first try was:
SELECT
DATE(timestamp) as day, SUM(units) as overall_units
FROM
tbl
GROUP BY
DATE(timestamp);
Normally this should do it. But sometimes there are days where is no record for a product. Nevertheless the units are still in the warehouse so they should be in the calculation.
For example:
We have 3 products. Cars, pens and wheels.
Records from 2012-10-20:
Cars => 5
Pens => 20
Wheels => 4
Records from 2012-10-21
Cars => 5
Wheels => 6
My query would give the following results:
2012-10-20 => 29
2012-10-21 => 11
But I want, that if there's no record for a product for the day it should use the record for this product which is the nearest one back in time.
So it should be:
2012-10-21 => 31
I hope you understand my needs.

Answer 1

SELECT
MAX (DATE(timestamp) ) as day, SUM(units) as overall_units
FROM tbl
update ::
SELECT max(day),sum(ou) from
( select DATE(timestamp) as day, SUM(units) as ou
FROM tbl
GROUP BY DATE(timestamp);
)
inner qry will return
2012-10-20 , 29
2012-10-21 , 11
and the final query will return
2012-10-21 , 40

Answer 2

SELECT
dd.ddate AS day
, SUM(t.units) as overall_units
FROM
( SELECT DISTINCT
product_id
FROM
tbl
) AS dp
CROSS JOIN
( SELECT DISTINCT
DATE(timestamp) AS ddate
FROM
tbl
) AS dd
JOIN
tbl AS t
ON t.id =
( SELECT
tt.id
FROM
tbl AS tt
WHERE
tt.product_id = dp.product_id
AND
tt.timestamp < dd.ddate + INTERVAL 1 DAY
ORDER BY tt.timestamp DESC
LIMIT 1
)
GROUP BY
dd.ddate ;

Answer 3

I think you should look into the DISTINCT() function.
The query could be something like: SELECT DISTINCT(product_id), * FROM tbl ORDER BY timestamp DESC; The use PHP to loop through your results and cumulate the units.