ACC2000: Creating Functions Similar to DFirst() and DLast()

By design, the DFirst() and DLast() functions always return the first and last record that you entered in the underlying table, not the first and last record as ordered by the table's index or the query's sort order. DFirst() and DLast() ignore indexes, even primary keys and sort orders.

This article shows you how to write custom domain functions that you can use in a way similar to DFirst() and DLast(). The DStart() and DEnd() custom domain examples in this article return the first and last records listed in a sorted query.

Microsoft provides programming examples for illustration only, without warranty either expressed or implied. This includes, but is not limited to, the implied warranties of merchantability or fitness for a particular purpose. This article assumes that you are familiar with the programming language that is being demonstrated and with the tools that are used to create and to debug procedures. Microsoft support engineers can help explain the functionality of a particular procedure, but they will not modify these examples to provide added functionality or construct procedures to meet your specific requirements.

CAUTION: If you follow the steps in this example, you modify the sample database Northwind.mdb. You may want to back up the Northwind.mdb file and follow these steps on a copy of the database.

To create and use custom domain functions similar to DFirst() and Dlast(), follow these steps:

1. Open the sample database Northwind.mdb.
2. Create a new module and enter the following two functions:

   '--------------------------------------
   ' Use DStart()instead of DFirst() to return
   ' the first sorted record in a domain.
   '--------------------------------------
   Function DStart(FieldName As String, DomainName As String, Optional _
     Criteria As Variant)
   
     Dim MyDB As DATABASE, MySet As Recordset
   
     ' Error out if there is no fieldname sent.
     If Len(FieldName) = 0 Then
        MsgBox "You Must Specify a Field name", , "DStart"
        Exit Function
     End If
   
     ' Error out if there is no domain sent.
     If Len(DomainName) = 0 Then
        MsgBox "You Must Specify a Domain name", , "DStart"
        Exit Function
     End If
   
     Set MyDB = CurrentDb()
     Set MySet = MyDB.OpenRecordset(DomainName, dbOpenDynaset)
   
     ' Apply a filter to the recordset if a criteria is sent.
     If Not IsMissing(Criteria) Then
        MySet.Filter = Criteria
        Set MySet = MySet.OpenRecordset()
     End If
   
     ' If there are no records, return the null, else return the value
     ' of the first record.
     If MySet.EOF Then
        DStart = Null
     Else
        MySet.MoveFirst
        DStart = MySet(FieldName)
     End If
   
     MySet.Close
     MyDB.Close
   End Function
   
   '-------------------------------------------
   'Use DEnd()instead of DLast() to return
   '  the last sorted record in a domain.
   '-------------------------------------------
   
   Function DEnd(FieldName As String, DomainName As String, Optional _
     Criteria As Variant)
   
     Dim MyDB As DATABASE, MySet As Recordset
   
     ' Error out if there is no fieldname sent.
     If Len(FieldName) = 0 Then
        MsgBox "You Must Specify a Field name", , "DEnd"
        Exit Function
     End If
   
     ' Error out if there is no domainname sent.
     If Len(DomainName) = 0 Then
        MsgBox "You Must Specify a Domain name", , "DEnd"
        Exit Function
     End If
   
     Set MyDB = CurrentDb()
     Set MySet = MyDB.OpenRecordset(DomainName, dbOpenDynaset)
   
     ' Apply a filter to the recordset if a criteria is sent.
     If Not IsMissing(Criteria) Then
        MySet.Filter = Criteria
        Set MySet = MySet.OpenRecordset()
     End If
   
     ' If there are no records, return the null, else return the value
     ' of the last record.
     If MySet.EOF Then
        DEnd = Null
     Else
        MySet.MoveLast
        DEnd = MySet(FieldName)
     End If
     MySet.Close
     MyDB.Close
   End Function
   
   					

3. Close and save the new module.
4. Create a new query based on the Orders table as follows:

      Query: Orders By Date
      ---------------------
      Type: Select Query
   
      Field: OrderID
      Table: Orders
      Field: OrderDate
      Table: Orders
      Sort: Descending
   					

5. Close and save the query as RecentOrders.
6. Open the Immediate window, type the following expression, and then press ENTER:

   ?DStart("OrderID","RecentOrders")
   						

   Note that the OrderID 11077 is returned, which is the first OrderID in the query.
7. Type the following expression in the Immediate window, and then press ENTER:

   ?DFirst("OrderID","RecentOrders")
   						

   Note that the OrderID 10314 (or 10248 in 7.0) is returned, which is the first OrderID entered in the Orders table, not the first in the RecentOrders query.

General Limitations to Custom Domain Functions

You cannot use Forms!FormName!ControlName or Form.ID in quotation marks. For example, instead of Note that the BuildCriteria() function is a Visual Basic function that creates a properly formed criteria string. You can use BuildCriteria() anywhere you concatenate criteria strings, such as the FindFirst method or when building custom SQL criteria.

For more information about the BuildCriteria() function, in the Visual Basic Editor, click Microsoft Visual Basic Help on the Help menu, type "BuildCriteria Method" in the Office Assistant or the Answer Wizard, and then click Search to view the topic.