Auto Increment number

One option would be to relax the requirement for batch number for each company to start at 1. Then, an auto-increment number would ensure batch number uniqueness.

Otherwise, an autonumber field is not appropriate here. Declare the field as type Numeric Long Integer and set its default value to be one greater than the maximum value of batch numbers for the current company. ex.:

Form Control property: DefaultValue
=1+DMax("BatchNumber","MyTable","MyCompanyID = " & Forms!MyCurrentForm!MyCompanyIDControlName)

Another method is to store the next number in a other table.  You may need to do this for a number of tables , so i usually store table ame, uniqueCd, and NextVal.

When i do this, i create a function to call just before i insert records to ensure there is no possibility that someone else can get the sAme value by using the dmax method.  I'm ony phone or i would provide the function, but instead of dmax i open a recordset and get the  nextVal and the reset it in two steps.

Or you could dispense with storing the customer sequence number altogether.  Your AutoNumber already provides a unique identifier in a specific order.  You can always calculate the customer-specific sequence numbers on demand, in your reports.

I have implemented the code, but keep getting and error when it runs:

Dim rst As DAO.Recordset

Set rst = CurrentDb.OpenRecordset("tbl_batches", dbOpenTable)
rst.AddNew
rst!customer_id = Me.customer_id
rst!batch_number = 1 + DMax("batch_number", "tbl_batches", "customer_id = " & Forms!frm_batches!customer_id)

rst.Update
rst.Close

Set rst = Nothing
End Sub

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It debugs to:

rst!batch_number = 1 + DMax("batch_number", "tbl_batches", "customer_id = " & Forms!frm_batches!customer_id)

with "Runtime Error '2471'
The Expression you entered as a query parameter produced this error: 'TR12043'

customer_id is a text field

Thanks

My bad - I got the syntax wrong.  I now have:

rst!batch_number = 1 + DMax("batch_number", "tbl_batches", "customer_id = '" & Me.customer_id & "'")

all working fine.

One thing I have realised is this.

If I'm adding a new customer and they currently do not have any Batches recorded in the database the DMax function will error out.  How would I add the first Batch ID to the database for new customers?

Also, how would I return this Batch ID to the original from so that the user can see what batch has been created?

I use something like the following.  I use a string as the UniqueCD to allow for either string or numeric values, and convert the Code that is passed into the function to a string for that reason.

Public Function fnNextNumber(TableName as string, Code as Variant) as Long

   Dim rst As DAO.Recordset
   Dim strSQL as string

   strSQL = "SELECT * FROM tbl_NextNumber " _
               & "WHERE [TableName] = '" & TableName & "'"
   Set rst = CurrentDb.OpenRecordset(strSQL, dbOpenTable, dbfailonerror)

   rst.FindFirst "[UniqueCD] = '" & cstr(Code) & "'"
   if rst.NoMatch then
        rst.AddNew
        rst!TableName = TableName
        rst!UniqueCD = cstr(Code)
        fnNextNumber = 1
        rst!NextVal = 2
   Else
        rst.Edit
       fnNextNumber = rst!NextVal
       rstNextVal = rstNextVal + 1
   endif
   rst.Update
   rst.close
   set rst = nothing

End Function

you need two different fields.  One field for the autoincrement values and another for the batch sequence values.  I implemented a sequencing in one of my applications in two passes.  The first pass added the rows.  I then queried the database to get the lowest autoincrement value for that client.  Using that value in an Update query, I was able to let the database to all of those sequence calculations.

From your description, it isn't clear to me how you want multiple batch rows numbered.