Select pairs of values based on condition in other column - PostgreSQL - sql

Question

I've been trying to solve an issue for the past couple of days, but couldn't figure out what the solution would be...
I have a table as the following:
+--------+-----------+-------+
| ShopID | ArticleID | Price |
+--------+-----------+-------+
| 1 | 3 | 150 |
| 1 | 2 | 80 |
| 3 | 3 | 100 |
| 4 | 2 | 95 |
+--------+-----------+-------+
And I woud like to select pairs of shop IDs for which the price of the same article is higher.
F.e. this should look like:
+----------+----------+---------+
| ShopID_1 | ShopID_2 |ArticleID|
+----------+----------+---------+
| 4 | 1 | 2 |
| 1 | 3 | 3 |
+----------+----------+---------+
... showing that Article 2 ist more expensive in ShopID 4 than in ShopID 2. Etc
My code so far looks as following:
SELECT ShopID AS ShopID_1, ShopID AS ShopID_2, ArticleID FROM table
WHERE table.ArticleID=table.ArticleID and table.Price > table.Price
But it doesn't give the result I am searching for.
Can anyone help me with this objective? Thank you very much.

Answer 1

The problem here is about calculating Top N items per Group.
Assuming you have the following data, in table sales.
# select * from sales;
shopid | articleid | price
--------+-----------+-------
1 | 2 | 80
3 | 3 | 100
4 | 2 | 95
1 | 3 | 150
5 | 3 | 50
With the following query we can create a partition for each ArticleId
select
ArticleID,
ShopID,
Price,
row_number() over (partition by ArticleID order by Price desc) as Price_Rank from sales;
This will result:
articleid | shopid | price | price_rank
-----------+--------+-------+------------
2 | 4 | 95 | 1
2 | 1 | 80 | 2
3 | 1 | 150 | 1
3 | 3 | 100 | 2
3 | 5 | 50 | 3
Then we simply select Top 2 items for each AritcleId:
select
ArticleID,
ShopID,
Price
from (
select
ArticleID,
ShopID,
Price,
row_number() over (partition by ArticleID order by Price desc) as Price_Rank
from sales) sales_rank
where Price_Rank <= 2;
which will result:
articleid | shopid | price
-----------+--------+-------
2 | 4 | 95
2 | 1 | 80
3 | 1 | 150
3 | 3 | 100
Finally, we can use crosstab function to get the expected pivot view.
select *
from crosstab(
'select
ArticleID,
ShopID,
ShopID
from (
select
ArticleID,
ShopID,
Price,
row_number() over (partition by ArticleID order by Price desc) as Price_Rank
from sales) sales_rank
where Price_Rank <= 2')
AS sales_top_2("ArticleID" INT, "ShopID_1" INT, "ShopID_2" INT);
And the result:
ArticleID | ShopID_1 | ShopID_2
-----------+----------+----------
2 | 4 | 1
3 | 1 | 3
Note:
You may need to call CREATE EXTENSION tablefunc; in case if you get the error function crosstab(unknown) does not exist.

Answer 2

This query should work:
SELECT t1.ShopID AS ShopID_1, t2.ShopID AS ShopID_2, t1.ArticleID
FROM <yourtable> t1 JOIN
<yourtable> t2
ON t1.ArticleID = t2.ArticleID AND t1.Price > t2.Price;
That is, you need a self-join and appropriate table aliases.