How to get only the recent joined pairs? - sql

Question

I've got two tables, where the second is connected to the first with multiple entries (1:n).
The first table has entries for an id_something (another table, but not important for now) and for every day date_day.
+-----------+
| Table ONE |
+----+------+-------+----------+----------+
| id | id_something | date_day | created |
+----+--------------+----------+----------+
| 1 | 666 | 2019-1-1 | 2019-1-1 |
| 2 | 666 | 2019-1-1 | 2019-7-7 |
| 3 | 123 | 2019-1-1 | 2019-1-1 |
+----+--------------+----------+----------+
The second table is connected to this id and contain key-value-pairs.
+-----------+
| Table TWO |
+--------+--+--+-----+
| id_one | foo | bar |
+--------+-----+-----+
| 1 | 1 | 20 |
| 1 | 2 | 21 |
| 2 | 1 | 30 |
| 2 | 2 | 31 |
| 2 | 3 | 32 |
| 3 | 1 | 10 |
+--------+-----+-----+
I want to query for all possible connections, so simple, it's a JOIN:
SELECT *
FROM one
LEFT JOIN two
ON two.id_one = one.id;
+----+--------------+----------+----------+--------+-----+-----+
| id | id_something | date_day | created | id_one | foo | bar |
+----+--------------+----------+----------+--------+-----+-----+
| 1 | 666 | 2019-1-1 | 2019-1-1 | 1 | 1 | 20 |
| 1 | 666 | 2019-1-1 | 2019-1-1 | 1 | 2 | 21 |
| 2 | 666 | 2019-1-1 | 2019-7-7 | 2 | 1 | 30 |
| 2 | 666 | 2019-1-1 | 2019-7-7 | 2 | 2 | 31 |
| 2 | 666 | 2019-1-1 | 2019-7-7 | 2 | 3 | 32 |
| 3 | 123 | 2019-1-1 | 2019-1-1 | 3 | 1 | 10 |
+----+--------------+----------+----------+--------+-----+-----+
Now as you see, I also have a created field. The id_something in conjunction with date_day could be the same - but I only want to have the most recent (created DESC) pairs with the second table.
So in this case, the query should return:
+----+--------------+----------+----------+--------+-----+-----+
| id | id_something | date_day | created | id_one | foo | bar |
+----+--------------+----------+----------+--------+-----+-----+
| 2 | 666 | 2019-1-1 | 2019-7-7 | 2 | 1 | 30 |
| 2 | 666 | 2019-1-1 | 2019-7-7 | 2 | 2 | 31 |
| 2 | 666 | 2019-1-1 | 2019-7-7 | 2 | 3 | 32 |
| 3 | 123 | 2019-1-1 | 2019-1-1 | 3 | 1 | 10 |
+----+--------------+----------+----------+--------+-----+-----+
But I can't get it to work... I tried to use DISTINCT or even a sub-query and a case construct, but it either doesn't work or is very imperformant. A group-by would also not return every joined pair, but just one single line for every id out of table one.
What am I missing to achieve my wished result?
(if no Oracle-specific synthax would be used, that would be a bonus.)

Answer 1

You can use analytic functions:
SELECT *
FROM (SELECT o.* ROW_NUMBER() OVER (PARTITION BY id ORDER BY created DESC) as seqnum
FROM one o
) o LEFT JOIN
two t
ON t.id_one = o.id
WHERE o.seqnum = 1;

Answer 2

use row_number() take the most recent id_something and then use join
select a.*,b.* from
(
select *,
row_number()over(partition by id_something order by created desc) rn from one
) a join two b ON b.id_one = a.id;
where rn=1