MAX Date <= to Date in another table - tsql

Question

We store exchange rates in a table and it is updated a few times a week for the latest exchange rates. I am trying to be able to take a date on a sales order, and assign it the exchange rate that was active at the time of the order. Below is an example of a listing of exchange rates and their effective dates, as well as sales orders, and what should be the effect rate (Exchage rate # time of order, which is what I want to pull in).
currate.effectivedate currate.currentrate
12/2/2012 0.55
12/7/2012 0.52
12/13/2012 0.54
12/19/2012 0.53
12/22/2012 0.56
orderhed.orderdate orderhed.ordernum Exchage Rate # date of order
12/4/2012 105 0.55
12/8/2012 111 0.52
12/9/2012 116 0.52
12/19/2012 120 0.53
12/28/2012 123 0.56
I have below an example of what I have been trying to do, but its not working. First, I can’t think of how to bring the currate.currentrate field in because of the MAX function on currate.effectivedate. Then for t_rate.effectivedate, I am getting just the MAX day of the exchange rate of the table period, not the max date that is <= the orderhed.orderdate. I need the MAX currate.effectivedate that is less than or equal to the orderhed.orderdate in order to get the effective date (and corresponding currentrate.currentrate) that was in effect at the time of the order. I am sorry if I am repeating myself, or if it doesn’t make sense. If it isn’t obvious by my sad little query below, I am extremely new to writing queries so any help I can get is greatly appreciated.
SELECT
orderhed.ordernum,
orderhed.orderdate,
orderdtl.orderline,
orderdtl.unitprice,
orderdtl.docunitprice,
orderdtl.discount,
orderdtl.docdiscount,
t_rate.effectivedate
FROM orderhed LEFT OUTER JOIN
(SELECT rate1.company, MAX(rate1.effectivedate) AS effectivedate, order1.orderdate
FROM currrate as rate1 INNER JOIN
orderhed AS order1 ON rate1.company = order1.company
WHERE (rate1.company = 'lotcol') AND (rate1.currencycode = 'usd')
GROUP BY rate1.company, order1.orderdate)
AS t_rate ON orderhed.company = t_rate.company AND t_rate.orderdate <= orderhed.orderdate
INNER JOIN
orderdtl ON orderhed.company = orderdtl.company
WHERE (orderhed.company = 'lotcol')
Update---
thanks #LastCoder you answer is what nudged me in the right direction. At the end of the day, this is exactly what worked:
SELECT orderhed.ordernum, orderhed.orderdate, orderdtl.orderline, orderdtl.unitprice, orderdtl.docunitprice, orderdtl.discount, orderdtl.docdiscount, t_rate.effectivedate,
t_rate.ordernum AS Expr1, t_rate1.currentrate
FROM orderhed INNER JOIN
(SELECT rate1.company, MAX(rate1.effectivedate) AS effectivedate, order1.orderdate, order1.ordernum
FROM currrate AS rate1 INNER JOIN
orderhed AS order1 ON rate1.company = order1.company AND rate1.effectivedate <= order1.orderdate
WHERE (rate1.company = 'lotcol') AND (rate1.currencycode = 'usd')
GROUP BY rate1.company, order1.orderdate, order1.ordernum) AS t_rate ON orderhed.ordernum = t_rate.ordernum AND orderhed.company = t_rate.company AND
t_rate.orderdate <= orderhed.orderdate INNER JOIN
(SELECT company, currencycode, effectivedate, currentrate
FROM currrate AS currrate_1
WHERE (company = 'lotcol') AND (currencycode = 'usd')) AS t_rate1 ON t_rate.company = t_rate1.company AND
t_rate.effectivedate = t_rate1.effectivedate INNER JOIN
orderdtl ON orderhed.company = orderdtl.company AND orderhed.ordernum = orderdtl.ordernum
WHERE (orderhed.company = 'lotcol')
ORDER BY orderhed.ordernum

Answer 1

SELECT o.orderdate
, ISNULL(r.effectivedate, (
SELECT MAX(effectivedate) FROM t_rate WHERE effectivedate < o.orderdate
)
FROM orderhed o
LEFT JOIN t_rate r ON o.orderdate = r.effectivedate
There's a minimal solution to your issue. You need to match the "orderdate" to the "effectivedate" that is equal to or closest less than it.