update 1 table from another table by next earliest date - sql-server

Question

I have 2 tables: budget and budget_rate:
Budget Table
resource period hours dollars
-------- ------ ----- -------
ADMIN03 01/31/16 160 8000
ADMIN03 02/28/16 150 7500
Rate Table
resource rate eff_date
-------- ---- --------
ADMIN03 50.00 01/01/16
ADMIN03 52.50 01/01/17
When the rates change in the rate table, I need to update the budget based on the rate that matches the resource name and is the first rate record earlier than the budget record.
Can this be accomplished with a single UPDATE?
Something like:
update b
set b.dollars = b.hours*r.rate
from
budget b join rate r on
b.resource = r.resource and
b.period >= r.eff_date

Answer 1

I assume rate table is realtive small, so I would recalculate it to have range columns.
with oRates as (
select resource,
rate,
eff_date,
ROW_NUMBER() over(partition by resource order by eff_date desc) rn
from Rates
),
pRates as (
select r1.resource,
r1.rate,
r1.eff_date from_date,
isnull(r2.eff_date,'2070-01-01') to_date
from oRates r1
left join oRates r2 on (r1.rn = r2.rn+1)
)
update b
set dollars = hours * r.rate
from Budget b
join pRates r on (b.resource = r.resource
and b.period >= from_date
and b.period < to_date)

Answer 2

One possible solution is using a computed column instead of some sort of manual update.
An example of how this could be done can be seen here: formula for computed column based on different table's column
For a working example with your data, you'd create a function like this:
CREATE FUNCTION dbo.ufn_BudgetDollars (#resource NVARCHAR(255), #date DATE, #hours INT)
RETURNS DECIMAL(10, 2)
AS BEGIN
DECLARE #out DECIMAL(10, 2);
SELECT #out = #hours * rate
FROM (
SELECT rate, ROW_NUMBER() OVER (ORDER BY eff_date DESC) rn
FROM tblRate
WHERE eff_date <= #date
AND resource = #resource) T
WHERE RN = 1;
RETURN #out;
END
GO
When you've created your function, you would want to drop and recreate the Dollars column on the budget table...
ALTER TABLE tblBudget DROP COLUMN Dollars;
ALTER TABLE tblBudget ADD Dollars AS dbo.ufn_BudgetDollars(resource, Period, Hours);
GO