Join two tables with a lesser than or equals condition in join - mysql

Question

Below two tables I have with sample data. Table A contains dollar rate (into Indian rupee) as per year, and Table B contains amount per year. I want to convert
dollar into rupee as per year.
Table A
Rate Year
47 2001
49 2003
55 2004
Table B
Amt Year
25$ 2001
34$ 2002
Question: for first record (year 2001) we have entry in both tables so we can do this easily by using below query
sel A.Rate * B.Amt
from A,
B
where B.year = A.year
But for second record (i.e. year 2002) we do not have entry in table A (which is rate table), so for these kind cases I want to use rate value from previous year (i.e.: 47 rupee from year 2001.)

Answer 1

Here is the solution :
select A.rate*B.amt
from A,B
where B.Year = (select max(year) from B where B.year <= A.year);

Answer 2

Oracle : use the LEAD analytic function so you can work out the validity period of each rate.
documentation for LEAD
This is my code :
SELECT
trx.*
,rates.rate_start_date
,rates.rate_end_date
,rates.rate
,trx.amount * rates.rate rup_amount
FROM
xxcjp_forex_trx trx
--this inline view works out the validity period of each rate by ordering all
--the rates and working out the start date of the next row. It uses analytic
--function LEAD
,(SELECT
xfr.rate_date rate_start_date
,xfr.rate
,xfr.currency
,(LEAD(xfr.rate_date) OVER (ORDER BY xfr.currency, xfr.rate_date))-1 rate_end_date
FROM
xxcjp_forex_rates xfr
) rates
WHERE 1=1
AND trx.trx_date BETWEEN rates.rate_start_date AND rates.rate_end_date
AND rates.currency = 'RUP'
ORDER BY
trx.trx_date
;
Based on this data :
CREATE TABLE XXCJP_FOREX_RATES
(rate_date DATE
,currency VARCHAR2(20)
,rate NUMBER
)
;
CREATE TABLE XXCJP_FOREX_TRX
(trx_date DATE
,currency VARCHAR2(20)
,amount NUMBER
)
;
INSERT INTO XXCJP_FOREX_RATES VALUES (TO_DATE('01/03/2016','DD/MM/YYYY'),'RUP',47) ;
INSERT INTO XXCJP_FOREX_RATES VALUES (TO_DATE('03/03/2016','DD/MM/YYYY'),'RUP',49) ;
INSERT INTO XXCJP_FOREX_RATES VALUES (TO_DATE('10/03/2016','DD/MM/YYYY'),'RUP',55) ;
INSERT INTO XXCJP_FOREX_TRX VALUES (TO_DATE('01/03/2016','DD/MM/YYYY'),'USD',10) ;
INSERT INTO XXCJP_FOREX_TRX VALUES (TO_DATE('02/03/2016','DD/MM/YYYY'),'USD',20) ;
INSERT INTO XXCJP_FOREX_TRX VALUES (TO_DATE('03/03/2016','DD/MM/YYYY'),'USD',30) ;
INSERT INTO XXCJP_FOREX_TRX VALUES (TO_DATE('04/03/2016','DD/MM/YYYY'),'USD',40) ;
INSERT INTO XXCJP_FOREX_TRX VALUES (TO_DATE('05/03/2016','DD/MM/YYYY'),'USD',50) ;
INSERT INTO XXCJP_FOREX_TRX VALUES (TO_DATE('06/03/2016','DD/MM/YYYY'),'USD',60) ;
INSERT INTO XXCJP_FOREX_TRX VALUES (TO_DATE('07/03/2016','DD/MM/YYYY'),'USD',70) ;
INSERT INTO XXCJP_FOREX_TRX VALUES (TO_DATE('08/03/2016','DD/MM/YYYY'),'USD',80) ;
INSERT INTO XXCJP_FOREX_TRX VALUES (TO_DATE('09/03/2016','DD/MM/YYYY'),'USD',90) ;
INSERT INTO XXCJP_FOREX_TRX VALUES (TO_DATE('10/03/2016','DD/MM/YYYY'),'USD',100) ;