使用多值Count（）数据创建Access表达式

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