Select last changed row in sub-query - sql

Question

I have a table product:
id | owner_id | last_activity | box_id
------------------------------------
1 | 2 | 12/19/2014 | null
2 | 2 | 12/13/2014 | null
3 | 2 | 08/11/2014 | null
4 | 2 | 12/11/2014 | 99
5 | 2 | null | 99
6 | 2 | 12/15/2014 | 99
7 | 2 | null | 105
8 | 2 | null | 105
9 | 2 | null | 105
The only variable that I have is owner_id.
I need to select all products of a user, but if the product is in a box then only latest one should be selected.
Sample output for owner = 2 is following:
id | owner_id | last_activity | box_id
------------------------------------
1 | 2 | 12/19/2014 | null
2 | 2 | 12/13/2014 | null
3 | 2 | 08/11/2014 | null
6 | 2 | 12/15/2014 | 99
7 | 2 | null | 105
I'm not able to find a way to select the latest product from a box.
My current query, which does not return correct value, but can be executed:
SELECT p.* FROM product p
WHERE p.owner_id = 2
AND (
p.box IS NULL
OR (
p.box IS NOT NULL
AND
p.id = ( SELECT MAX(pp.id) FROM product pp
WHERE pp.box_id = p.box_id )
)
I tried with dates:
SELECT p.* FROM product p
WHERE p.owner_id = 2
AND (
p.box IS NULL
OR (
p.box IS NOT NULL
AND
p.id = ( SELECT * FROM (
SELECT pp.id FROM product pp
WHERE pp.box_id = p.box_id
ORDER BY last_activity desc
) WHERE rownum = 1
)
)
Which gives error: p.box_id is undefined as it's inside 2nd subquery.
Do you have any ideas how can I solve it?

Answer 1

The ROW_NUMBER analytical function might help with such queries:
SELECT "owner_id", "id", "box_id", "last_activity" FROM
(
SELECT "owner_id", "id", "box_id", "last_activity",
ROW_NUMBER()
OVER (PARTITION BY "box_id" ORDER BY "last_activity" DESC NULLS LAST) rn
-- ^^^^^^^^^^^^^^^
-- descending order, reject nulls after not null values
-- (this is the default, but making it
-- explicit here for self-documentation
-- purpose)
FROM T
WHERE "owner_id" = 2
) V
WHERE rn = 1 or "box_id" IS NULL
ORDER BY "id" -- <-- probably not necessary, but matches your example
See http://sqlfiddle.com/#!4/db775/8
there can be nulls as a value. If there are nulls in all products inside a box, then MIN(id) should be returned
Even if is is probably not a good idea to rely on id to order things is you think you need that, you will have to change the ORDER BY clause to:
... ORDER BY "last_activity" DESC NULLS LAST, "id" DESC
-- ^^^^^^^^^^^

Answer 2

Use exists
SELECT
p.*
FROM
product p
WHERE
p.owner_id = 2 AND
( p.box IS NULL OR
(
p.box IS NOT NULL AND
NOT EXISTS
(
SELECT
pp.id
FROM
product pp
WHERE
pp.box_id = p.box_id AND
pp.last_activity > p.last_activity
)
)
)

Answer 3

You can use union to first get all rows where box_is null and than fetch rows with max id and date where box_id is not null:
SELECT * FROM
(
SELECT id,owner_id,last_activity,box_id FROM product WHERE owner_id = 2 AND box_id IS NULL
UNION
SELECT MAX(id),owner_id,MAX(last_activity),box_id FROM product WHERE owner_id = 2 AND box_id IS NOT NULL GROUP BY owner_id, box_id
) T1
ORDER BY
id