Optimising a VBA procedure that iterates over two different tables of data - sql

Question

I have two sql tables pricetbl and paymenttbl. Due to the fact that prices for item change everyday, I need to update the paymenttbl with prices of the right time. If a transaction is made after the date-effective of a product's price, then the transaction price in the paymenttbl is the same as the latest product's price (as long as the date-effective is smaller than transaction date). I don't only want to limit myself here, I want to iterate over the whole table to make changes to the data so every single transaction has the proper price.
PriceTbl
pID| Price| Date Effective
---------------------------
1 | 10 | 01-12-2014
2 | 20 | 01-02-2015
3 | 20 | 02-12-2014
2 | 40 | 20-03-2015
1 | 50 | 02-03-2015
4 | 34 | 20-02-2015
1 | 40 | 25-05-2015
PaymentTbl
payID | pID | transDate | Price
--------------------------------
1 | 1 | 02-12-2014| 05
2 | 1 | 04-03-2015| 10
3 | 2 | 21-04-2015| 35
4 | 3 | 03-12-2014| 15
Expected table paymenttbl after running query
payID | pID | transDate | Price
--------------------------------
1 | 1 | 02-12-2014| 10
2 | 1 | 04-03-2015| 50
3 | 2 | 21-04-2015| 40
4 | 3 | 03-12-2014| 20
What will be the fastest and easiest way to retrieve and update data? My current way takes up to like 30 minutes to update the table since I iterate over the paymenttbl then I iterate over the pricetbl and compare it to each item in the paymenttbl.
My Code:
Function updateWorktoDateEff()
Dim rt As DAO.Recordset
Dim wt As DAO.Recordset
Dim db As DAO.Database
Dim sqlrt As String
Dim sqlwt As String
Set db = CurrentDb
sqlrt = "select * from pricetbl where (dailyrate IS NOT NULL) and (dateeffective is not null) order by id, dailyrate"
sqlwt = "select * from paymenttbl where (id > 0) order by id, date"
Set rt = db.OpenRecordset(sqlrt)
Set wt = db.OpenRecordset(sqlwt)
Do While Not wt.EOF
Do While Not ratetable.EOF
If rt!ID = wt!ID Then
If rt <= wt!Date Then
wt.Edit
wt!Rate = ratetable!dailyrate
wt.Update
End If
ElseIf ratetable!ID > worktable!ID Then
Exit Do
End If
rt.MoveNext
Loop
rt.MoveFirst
wt.MoveNext 'move to the next worktbl record
Loop
End Function
simply iterating/retrieving the data wouldn't do it. I will also need to be able to alter it.

Answer 1

The most efficient way to perform this update is not to use recordsets and go row-by-row, but to use SQL to do a set-based update. I tried writing a single update query to get there, but MS-Access got fussy about the query not being updateable, so I instead wrote two queries: one to write to a temporary table (tempNewPrices) and one to update PaymentTbl using that temporary table.
Here's the first query to create the temporary table that includes the new column NewPrice:
SELECT PaymentTbl.payID, PaymentTbl.pID, PaymentTbl.transDate, PaymentTbl.Price,
(
SELECT [Price]
FROM PriceTbl pr
WHERE pr.pID = PaymentTbl.pID
AND pr.[Date Effective] = (
SELECT Max([Date Effective])
FROM PriceTbl pr2
WHERE pr2.pID = pr.pID
AND pr2.[Date Effective]< PaymentTbl.[transDate]
)
) AS NewPrice INTO tempNewPrices
FROM PaymentTbl;
and here's the update query to change the Price value in PaymentTbl:
UPDATE tempNewPrices
INNER JOIN PaymentTbl ON tempNewPrices.payID = PaymentTbl.payID
SET PaymentTbl.Price = [NewPrice];
This should be much more efficient than looping through recordsets.