SQL query applying currency conversion rate - sql

Question

Please help to compose an SQL query having a table with currency conversion rates that should be applied to the other table with business data, for example:
currency_rates (conversion rate is given for every beginning of the month, starting from some moment)
currency_code
rate against USD
date
CAD
1.354
2022-11-01
CAD
1.3445
2022-12-01
CAD
1.3573
2023-01-01
business_data (fees are in USD, aggregated by name and date)
name
sum(fee) in USD
date
aaa
92.52
2021-10-10
bbb
76.18
2022-11-11
ccc
113.79
2022-12-12
ddd
133.42
2023-02-02
The expected result should be the following, assuming that aggregated fee dates lay in the range of conversion rate dates (the actual result of the multiplication isn't matter here, I show x*y just to describe an idea):
name
sum(fee) in USD * CAD conversion rate
date
aaa
92.52 * 1.354 (applied first existing rate)
2021-10-10
bbb
76.18 * 1.354
2022-11-11
ccc
113.79 * 1.3445
2022-12-12
ddd
133.42 * 1.3573 (applied last existing rate)
2023-02-02
We use Snowflake DB.
Don't hesitate to ask questions or add suggestions, thank you!

Answer 1

You can join the tables on year and month
select a.*, b.currency_rate --multiply as you wish
from business_data a
join currency_date b on
year(a.date)=year(b.date) and
month(a.date)=month(b.date)
Or more concisely,
select a.*, b.currency_rate --multiply as you wish
from business_data a
join currency_date b on trunc(a.date,'month')=b.date
As Adrian notes, my solution above doesn't account for the fact that your currency_data table does not have all the months that exist in your business_data table so let's fix that separately instead of complicating the code above.
Your sample data doesn't establish any relationship between currency_code and business data. For that reason I am going to use hardcoded 'CAD' for currency. If your business_data table has a column that ties to currency_code, you can easily modify this solution by incorporating that column in the full outer join and select statement. Also, you can switch up the order of columns inside coalesce to handle situations where you have both "last existing" and "first existing" rate to choose from.
with cte as
(select 'CAD' as currency_code, --ideally, this would a column that joins to currency data table even though your sample data doesn't indicate such a relationship
b.rate,
coalesce(trunc(a.business_date, 'month'),currency_date) as currency_date
from business_data a
full outer join currency_data b on b.currency_date=trunc(a.business_date, 'month') )
select currency_code ,
currency_date,
coalesce(rate,
lead(rate) ignore nulls over (partition by currency_code order by currency_date),
lag(rate) ignore nulls over (partition by currency_code order by currency_date)) as rate
from cte
order by currency_date

Answer 2

UNION ALL
LAG()
LEAD()
COALESCE()
JOIN
BETWEEN
WITH CURRENCY_RATES AS(
SELECT 'CAD' CURRENCY_CODE, 1.354 RATE, '2022-11-01'::DATE CURRENCY_DATE
UNION ALL SELECT 'CAD' CURRENCY_CODE, 1.3445 RATE, '2022-12-01'::DATE CURRENCY_DATE
UNION ALL SELECT 'CAD' CURRENCY_CODE, 1.3573 RATE, '2023-01-01'::DATE CURRENCY_DATE)
,CURRENCY_RATES_ENHANCED AS (
SELECT
CURRENCY_CODE
, RATE
, CURRENCY_DATE
, IFF(LAG(CURRENCY_DATE)OVER(PARTITION BY CURRENCY_CODE ORDER BY CURRENCY_DATE) IS NULL,'1900-01-01',CURRENCY_DATE) CURRENCY_START_DATES
, COALESCE(LEAD(CURRENCY_DATE)OVER(PARTITION BY CURRENCY_CODE ORDER BY CURRENCY_DATE ASC),CURRENT_DATE+999)-INTERVAL '1 SEC' CURRENCY_END_DATES
FROM CURRENCY_RATES
)
,BUSINSESS_DATA AS ( SELECT 'aaa' NAME, 92.52 FEE_IN_USD, '2021-10-10'::DATE BUSINESS_DATE
UNION ALL SELECT 'bbb' NAME, 76.18 FEE_IN_USD, '2022-11-11'::DATE BUSINESS_DATE
UNION ALL SELECT 'ccc' NAME, 113.79 FEE_IN_USD, '2022-12-12'::DATE BUSINESS_DATE
UNION ALL SELECT 'ddd' NAME, 133.42 FEE_IN_USD, '2023-02-02'::DATE BUSINESS_DATE)
SELECT
NAME,
FEE_IN_USD||' * '||RATE||
IFF(CURRENCY_START_DATES='1900-01-01'::DATE AND DATE_TRUNC('MONTH',B.BUSINESS_DATE)<>C.CURRENCY_DATE ,' (applied first existing rate)'
,IFF(CURRENCY_END_DATES=CURRENT_DATE+999-INTERVAL '1 SEC',' (applied last existing rate)',null)) VOLIA
,B.BUSINESS_DATE
FROM
BUSINSESS_DATA B
JOIN CURRENCY_RATES_ENHANCED C
ON BUSINESS_DATE BETWEEN CURRENCY_START_DATES AND CURRENCY_END_DATES
;