Access SQL Query from another DB

You prefer not to use a link to the table in the external database, but that choice is a complication when you want to use DAvg. However, since you're doing this with VBA code, you can ditch DAvg and do what you need in 2 steps:

1. First retrieve the average from the external table.
2. Use that step #1 average in your UPDATE.

For step #1, test this as a new query in the Access query designer ...

SELECT Avg(Field1InExternalDB)
FROM TableInExternalDB IN 'C:\VB\ExternalDB.accdb'
WHERE Field2InExternalDB=Year(Date());

Assuming that query returns the correct value, adapt your VBA code to retrieve the same value.

Dim db As DAO.database
Dim strSelect As String
Dim varAvg As Variant
strSelect = "SELECT Avg(Field1InExternalDB)" & vbCrLf & _
    "FROM TableInExternalDB IN 'C:\VB\ExternalDB.accdb'" & vbCrLf & _
    "WHERE Field2InExternalDB=Year(Date());"
'Debug.Print strSelect
Set db = CurrentDb
varAvg = db.OpenRecordset(strSelect)(0)
Debug.Print Nz(varAvg, 0) ' see note

Note that query will return Null when no rows include Field2InExternalDB values which match the current year. That is why varAvg is declared as Variant. Later Nz(varAvg, 0) will give you zero instead of Null.

Then you can use a parameter query for your UPDATE and supply Nz(varAvg, 0) as the parameter value.

Dim qdf As DAO.QueryDef
Dim strUpdate As String
strUpdate = "UPDATE TableInCurrentDB" & vbCrLf & _
    "SET [Field1InCurrentDB]=[pAvg]" & vbCrLf & _
    "WHERE [Field2InCurrentDB]='1';"
'Debug.Print strUpdate
Set qdf = db.CreateQueryDef(vbNullString, strUpdate)
qdf.Parameters("pAvg") = Nz(varAvg, 0)
qdf.Execute dbFailOnError
Set qdf = Nothing
Set db = Nothing