Aggregate multiplicate function - ms-access

Question

I have a table of the following:
1 X 10
2 X 30
3 Y 5
4 Y 2
...etc
I need to turn it into:
X 300 //(10 * 30)
Y 10 //(5 * 2)
so I'm in fact looking for a kind of a multiplication function that I could use like sum, avg and stuff... does this exist guys?
for instance:
select field2, **multiply**(field3)
from t
group by field2
Thanks

Answer 1

Ideally, Access SQL would have a PRODUCT aggregate function available, but it doesn't. We can however simulate it by remembering what we learned about logarithms at school (or not...), and remembering that the anti-log of the sum of logs is equal to the product:
SELECT field2, EXP(Sum(LOG(Field3))) AS ProductOfField3
FROM t
GROUP BY Field2
Note that whereas a true PRODUCT function would simply return 0 for a group if there are any zero values, this solution will fail if there are any zero values, so watch out for that. Also, this approach won't work if there are any negative values.
To deal with zeroes we could do this:
SELECT
field2,
EXP(Sum(LOG(IIf(Field3 = 0, 1, Field3)))) AS ProductOfField3,
MIN(ABS(Field3)) AS MinOfAbsField3
FROM t
GROUP BY Field2
and then disregard the ProductOfField3 value for any row where MinOfAbsField3 is zero (as this indicates a group containing a zero, thus the 'true' product should be 0)
To deal with negative values we could further do this:
SELECT
field2,
EXP(Sum(LOG(IIf(Field3 = 0, 1, ABS(Field3))))) AS ProductOfField3,
MIN(ABS(Field3)) AS MinOfAbsField3,
SUM(IIf(Field3 < 0, 1, 0)) AS SumOfNegativeIndicator
FROM t
GROUP BY Field2
and interpret the results with these rules:
If MinOfAbsField3 is zero, disregard ProductOfField3 for that row - the product is zero
Otherwise, the required answer for a given row is ProductOfField3, negated if SumOfNegativeIndicator is odd in that row

Answer 2

You can define following two functions:
Public Function ResetProd() As Boolean
Call Prod(Null)
ResetProd = True
End Function
Public Function Prod(nNumber As Variant) As Double
Static nPrevNumber As Double
If IsNull(nNumber) Then
nPrevNumber = 1
Else
nPrevNumber = nPrevNumber * nNumber
End If
Prod = nPrevNumber
End Function
...and use them as follows:
SELECT PROD(_column_for_wich_you_want_product_) FROM _your_table_
WHERE RESETPROD()