Dynamic Calculated Field in VBA Access - sql

Question

I'm trying to add a field into an existing table. The field is calculated using two variables 'myPrice', which is the price of a record at a certain date, and 'previousPrice', which is the price of the same part from the first record, only a from the month previous. I'm using a loop to iterate through the entire recordset of prices, thereby altering the two variables each time. My code is as follows:
Function priceDifference()
Dim currentPrice As Currency
Dim previousPrice As Currency
Dim recordDate As Date
Dim previousDate As Date
Dim dbsASSP As DAO.Database
Dim priceTable As DAO.TableDef
Dim diffFld As DAO.Field2
Dim rstCurrentPrice As DAO.Recordset
Dim rstPreviousPrice As DAO.Recordset
Dim PN As String
Set dbsASSP = CurrentDb
strSQL = "SELECT * FROM qryPrice"
Set rstCurrentPrice = dbsASSP.OpenRecordset(strSQL, dbOpenDynaset)
Set priceTable = dbsASSP.TableDefs("tblPrice")
Set diffFld = priceTable.CreateField("Difference", dbCurrency)
If Not (rstCurrentPrice.EOF) Then
rstCurrentPrice.MoveFirst
Do Until rstCurrentPrice.EOF = True
PN = rstCurrentPrice![P/N]
recordDate = rstCurrentPrice![myDate]
previousDate = Format(DateAdd("m", -1, recordDate), "M 1 YYYY")
myPrice = rstCurrentPrice!Price
strPreviousSQL = "SELECT * FROM qryPrice WHERE [MyDate] = #" & previousDate & "# AND [Type] = 'Net Price' AND [P/N] = " & PN & ""
Set rstPreviousPrice = dbsASSP.OpenRecordset(strPreviousSQL, dbOpenDynaset)
myCodeName = rstCurrentPrice!CodeName
If DCount("[P/N]", "qryPrice", "[MyDate] = #" & previousDate & "# And [P/N] = " & PN) <> 0 Then
previousPrice = rstPreviousPrice!Price
Else
previousPrice = myPrice
End If
rstCurrentPrice.MoveNext
Loop
Else
MsgBox "Finished looping through records."
End If
diffFld.Expression = myPrice - previousPrice
rstCurrentPrice.Close
rstPreviousPrice.Close
priceTable.Fields.Append diffFld
End Function
Syntactically, it works. The calculated field, however, does not give me the correct values and I cannot figure out why, although I imagine it has something to do with the dynamic formula.
Any help is appreciated. Thanks!!

Answer 1

Your code can't work as you append a number to the field instead of a formula, but you should avoid calculated fields
Use a query:
SELECT
nPrice.[P/N],
nPrice.Price,
nPrice.MyDate,
oPrice.MyDate,
nPrice.Price-Nz(oPrice.Price,nPrice.Price) AS diff
FROM (
SELECT
[P/N],
Price,
MyDate,
Format(DateAdd("m",-1,MyDate),"yyyymm") as previousMonthYear
FROM
tblPrice) AS nPrice
LEFT JOIN (
SELECT
[P/N],
Price,
MyDate,
Format(MyDate,"yyyymm") AS monthYear
FROM
tblPrice) AS oPrice
ON
nPrice.[P/N] = oPrice.[P/N]
AND nPrice.previousMonthYear = oPrice.monthYear
This calculates the difference dynamic and you don't need to store it.
I assume there is max one price per month, otherwise you need to filter the subqueries or just calculate the difference to the last price before.
If the query is too slow ([P/N] and MyDate need an index) , you can still use it to update a field the table tblPrice, but this has to be updated every time a price is updated or inserted in tblPrice

Answer 2

I think you can avoid your code loop by using this query
SELECT A.qryPrice, A.MyDate, B.qryPrice, B.MyDate
FROM qryPrice A LEFT OUTER JOIN qryPrice B
ON A.[P/N] = B.[P/N]
AND B.MyDate = DATEADD(month, -1, A.MyDate)