Create an Access expression using multi-value Count() data - sql

Question

The data I have is in a few tables. Table 1 has:
[invoice] (key),
[invoice total], and
[associated assets] (multi-value linked to [assets] in table 2).
Table two includes:
[Assets] (key), and
other asset data fields.
What I need to do is take the invoice total and divide it by the number of assets that go into that price to determine the cost of each asset. I have used the count() function in a query to get the number of assets for each invoice, but I can't seem to use the data anywhere else.
I have created another query to use the expression
= [invoice total]/count([associated assets]
In queries, I keep getting an aggregate error and can't seem to make this work. I would like for the expression result to populate a field in table 1. I need help either setting up a working query to pull the data from, or a form item that will pull the data and auto-populate the field.

Answer 1

For a table named [Invoices]
invoice invoice total associated assets
------- ------------- -----------------
1 $3.00 bicycles
2 $5.00 bicycles, ham
3 $1.00
where [associated assets] is a multi-value lookup field against the table [Assets]
AssetID AssetName
------- ---------
1 bicycles
2 ham
The following query
SELECT
i.invoice,
i.inv_tot AS [invoice total],
i.assetCount,
IIf(i.assetCount=0, NULL, i.[invoice total]/i.assetCount) AS avgCostPerAsset
FROM
(
SELECT
Invoices.invoice,
Min(Invoices.[invoice total]) AS inv_tot,
Count(Invoices.[associated assets].Value) AS assetCount
FROM Invoices
GROUP BY Invoices.invoice
) i
produces the following result
invoice invoice total assetCount avgCostPerAsset
------- ------------- ---------- ---------------
1 $3.00 1 3
2 $5.00 2 2.5
3 $1.00 0