How to do one-to-one inner join - sql

Question

I've a transaction table of purchased and returned items, and I want to match a return transaction with the transaction where that corresponding item was purchased. (Here I used the same item ID and amount in all records for simplicity)
trans_ID
date
item_ID
amt
type
1
2022-01-09
100
5000
purchase
2
2022-01-07
100
5000
return
3
2022-01-06
100
5000
purchase
4
2022-01-05
100
5000
purchase
5
2022-01-04
100
5000
return
6
2022-01-03
100
5000
return
7
2022-01-03
100
5000
purchase
8
2022-01-02
100
5000
purchase
9
2022-01-01
100
5000
return
Matching conditions are:
The return date must be greater than or equal the purchase date
The return and purchase transactions must relate to the same item's ID and same transaction amount
For each return, there must be only 1 purchase matched to it (In case there are many related purchases, choose one with the most recent purchase date. But if the most recent purchase was already used for mapping with another return, choose the second-most recent purchase instead, and so on.)
From 3), that means each purchase must be matched with only 1 return as well.
The result should look like this.
trans_ID
date
trans_ID_matched
date_matched
2
2022-01-07
3
2022-01-06
5
2022-01-04
7
2022-01-03
6
2022-01-03
8
2022-01-02
This is what I've tried.
with temp as (
select a.trans_ID, a.date
, b.trans_ID as trans_ID_matched
, b.date as date_matched
, row_number() over (partition by a.trans_ID, a.date order by b.date desc) as rn1
from
(
select *
from transaction_table
where type = 'return'
) a
inner join
(
select *
from transaction_table
where type = 'purchase'
) b
on a.item_ID = b.item_ID and a.amount = b.amount and a.date >= b.date
)
select * from temp where rn = 1
But what I got is
trans_ID
date
trans_ID_matched
date_matched
2
2022-01-07
3
2022-01-06
5
2022-01-04
7
2022-01-03
6
2022-01-03
7
2022-01-03
Here, the trans ID 7 shouldn't be used again in the last row as it has been already matched with trans ID 5 in the row 2. So is there any way to match trans ID 6 with 8 (or any way to tell SQL not to use the already-used one like the purchase 7) ?

Answer 1

I created a fiddle, the result seem OK, but it's up to you to test if this is OK on all situtations..... 😉
WITH cte as (
SELECT
t1.trans_ID,
t1.[date],
t1.item_ID,
t1.amt,
t1.[type],
pur.trans_ID trans_ID_matched,
pur.[date] datE_matched,
jojo.c
FROM table1 t1
CROSS APPLY (
SELECT
trans_ID,
item_ID,
[date],
amt
FROM table1 pur
WHERE pur.[type] = 'purchase' and t1.[type]='return'
and pur.item_ID = t1.item_ID
and pur.amt = t1.amt
and pur.[date] <= t1.[date]
) pur
CROSS APPLY (
SELECt count(*) as c FROM table1 WHERE trans_ID> t1.trans_ID and trans_ID<pur.trans_ID
) jojo
where jojo.c <=2
)
select
trans_ID,
[date],
item_ID,
amt,
CASE WHEN min(c)=0 then min(trans_ID_matched) else max(trans_ID_matched) end
from cte
group by
trans_ID,
[date],
item_ID,
amt
order by trans_ID;
DBFIDDLE
The count(*) detects the distance between the selected trans_ID from the return and the purchase.
This might go wrong the are more than 2 adjacent 'returns'... (I am afraid it will break, so I did not test this 😢).
But is's a nice problem. Hopefully this will give you any other ideas to find the correct sulution!