Access SQL - Get last available value - sql

Question

scenario:
Table 1 (dates)
01/01/12
02/01/12
Table 2 (counts)
01/01/12 | Item 1 | 10
01/01/12 | Item 2 | 15
03/01/12 | Item 1 | 5
I have to join the two tables, not leaving gaps in dates:
An OUTER JOIN did the trick.
I want to have the last available number if that date is missing in Table 2.
E.g.
02/01/12 | Item 1 | 10
02/01/12 | Item 2 | 15
I am struggling on this. Any hints?

Answer 1

A query can transform, and join data, but it cannot generate new data (rows) that were not there to begin with. In your case if the 02/01/12 data row is missing you aren't going to be able to create rows on the fly to fill in the gaps in your query results.
To fix, create a 3rd table or sub-query that contains all of the lookup date segments that you need in your results, and include that in your outer join on the outer query.

Answer 2

Okay, so we have [Table1]
Date
----------
2013-01-01
2013-02-01
2013-03-01
and [Table2]
Date Item Qty
---------- ------ ---
2013-01-01 Item 1 10
2013-01-01 Item 2 15
2013-03-01 Item 1 5
We can start by creating a saved query in Access named [DesiredRows] to give us the complete set of Date/Item values we want to see:
SELECT Table1.Date, i.Item
FROM
Table1,
(SELECT DISTINCT Item FROM Table2) AS i
...returning
Date Item
---------- ------
2013-01-01 Item 1
2013-02-01 Item 1
2013-03-01 Item 1
2013-01-01 Item 2
2013-02-01 Item 2
2013-03-01 Item 2
Now we can use that query as part of a saved query named [LastAvailableDates] to give us the last available date in Table2 for each of those pairs
SELECT
DesiredRows.Date,
DesiredRows.Item,
MAX(Table2.Date) AS LastAvailableDate
FROM
DesiredRows
INNER JOIN
Table2
ON Table2.Item = DesiredRows.Item
AND Table2.Date <= DesiredRows.Date
GROUP BY DesiredRows.Date, DesiredRows.Item
...returning
Date Item LastAvailableDate
---------- ------ -----------------
2013-01-01 Item 1 2013-01-01
2013-01-01 Item 2 2013-01-01
2013-02-01 Item 1 2013-01-01
2013-02-01 Item 2 2013-01-01
2013-03-01 Item 1 2013-03-01
2013-03-01 Item 2 2013-01-01
Finally, we can use that query in a query to retrieve the final results
SELECT
LastAvailableDates.Date,
LastAvailableDates.Item,
Table2.Qty
FROM
LastAvailableDates
INNER JOIN
Table2
ON Table2.Date = LastAvailableDates.LastAvailableDate
AND Table2.Item = LastAvailableDates.Item
...returning
Date Item Qty
---------- ------ ---
2013-01-01 Item 1 10
2013-01-01 Item 2 15
2013-02-01 Item 1 10
2013-02-01 Item 2 15
2013-03-01 Item 1 5
2013-03-01 Item 2 15