I need to separate a PostgreSQL table in this intercalated format - sql

Question

I have this query in Postgres:
Select
"Charges"."saleAmount", "Charges"."buyAmount", "Operations"."id"
From
"Charges"
Left Join
"Operations" On "Operations"."id" = "Charges"."operationsId"
Order By
"Operations"."id"
saleAmount
buyAmount
id
200
NULL
id1
300
500
id2
0
100
id3
I need to transform it: Add a new column type depending on saleAmount > 0 or buyAmount > 0, and separate into two rows when I have both saleAmount and buyAmount in the same row.
saleAmount
buyAmount
id
type
200
NULL
id1
sale
300
0
id2
sale
0
500
id2
buy
0
100
id3
buy
How do I transform my table into this format?
Column type can be made with:
(CASE
WHEN "saleAmount" > 0 THEN 'sale'
WHEN "buyAmount" > 0 THEN 'buy'
END) as "type"

Answer 1

You can use UNION ALL to create two rows out of one. E.g.:
Select c."saleAmount", c."buyAmount", o."id", c.type
From
(
Select
"saleAmount",
Case When "buyAmount" > 0 Then 0 Else "buyAmount" End As "buyAmount",
'sale' as type
From "Charges"
Where "saleAmount" > 0
Union All
Select
Case When "saleAmount" > 0 Then 0 Else "saleAmount" End As "saleAmount",
"buyAmount",
'buy' as type
From "Charges"
Where "buyAmount" > 0
) c
Left Join "Operations" o On o."id" = c."operationsId"
Order By o."id";
The join of the Operations table seems superfluous by the way. Either the Charges has an operationsId, then it links to an Operations row with the same ID or it doesn't have an operationsId, then it doesn't link to Operations row. So why not just show the "Charges"."operationsId" instead of joining to the Operations table just to show the same ID?

Answer 2

You can join to a values table constructor and use a case expression to determine how many rows qualify for the join:
select t.*
from t
join (
values(1),(2)
)x(r) on r <= case
when Coalesce(saleamount, 0) > 0
and Coalesce(buyAmount, 0) > 0
then 2 else 1 end;

Answer 3

I think you can use union all.
select
c.saleAmount,
0 buyAmount,
o.id,
'sale'
from Charges c
Left Join "Operations" o On o."id" = c."operationsId"
where isnull(c.saleAmount) > 0
union all
select
0 saleAmount,
c.buyAmount,
o.id,
'buy'
from Charges c
Left Join "Operations" o On o."id" = c."operationsId"
where isnull(c.buyAmount,0) > 0

Answer 4

In Postgres you can unpivot the two columns to rows with values and a lateral join, then filter out unwanted rows in the where clause:
select c.*, t.type
from charges c
cross join lateral (
values (c.saleamount, 'sale'), (c.buyamount, 'buy')
) t(amount, type)
where t.amount > 0
It is not obvious what the purpose of the left join in the original query is, so I left it apart - but you can easily add it to the query if needed.