ACC: Referring to a Field in the Previous or Next Record

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Summary

Advanced: Requires expert coding, interoperability, and multiuser skills.


This article shows you two methods that you can use to obtain values from the previous or the next record for use in calculations. The first method uses the DLookup()function in an expression; the second method uses two user-defined functions.


NOTE: A demonstration of the technique used in this article can be seen in the sample file, Qrysmp97.exe. For information about how to obtain this sample file, please see the following article in the Microsoft Knowledge Base:
182568 ACC97: Microsoft Access 97 Sample Queries Available in Download Center

More Information

Using DLookup()

NOTE: This technique assumes you have a table with an ID field of a Number data type, and the ID values are not missing any number in sequential order. If your table does not meet these criteria, then you should use the "Using Code" method described later in this article.


You can use the following sample DLookup() expressions to obtain values from a field in the previous or next record of a form, a report, or a query.

In a Form:

To obtain a value from a field in the previous record, type the following line for the text box's ControlSource property:

   =DLookUp("[Field]","Table","[ID]=Forms![Form1]![ID]-1")
To obtain a value from a field in the next record, type the following line for the text box's ControlSource property:
   =DLookUp("[Field]","Table","[ID]=Forms![Form1]![ID]+1")

In a Report:

To obtain a value from a field in the previous record, type the following line for the text box's ControlSource property:
   =DLookUp("[Field]","Table","[ID]=Reports![Report1]![ID]-1")
To obtain a value from a field in the next record, type the following line for the text box's ControlSource property:
   =DLookUp("[Field]","Table","[ID]=Reports![Report1]![ID]+1")

In a Query:

To obtain a value from a field in the previous record, type the following line in the Field row of the query grid:
   Expr1: DLookUp("[Field1]","Table1","[ID]=" & [ID]-1)
To obtain a value from a field in the next record, type the following line in the Field row of the query grid:
   Expr1: DLookUp("[Field1]","Table1","[ID]=" & [ID]+1)
NOTE: In these sample expressions, the "-1" and "+1" indicate the previous and next records. When the current record is the first record in the recordset, the "-1" returns a Null value because there is no previous record. Likewise, when the current record is the last record in the recordset, the "+1" returns a Null. If you want to return a value from a record other than the next or previous one, you can specify a different number, for example, "-3" for the third previous record.

Using Code

This section assumes that you are familiar with Visual Basic for Applications and with creating Microsoft Access applications using the programming tools provided with Microsoft Access. For more information about Visual Basic for Applications, please refer to your version of the "Building Applications with Microsoft Access" manual.


NOTE: Visual Basic for Applications is called Access Basic in Microsoft Access version 1.x and Microsoft Access version 2.0.


To retrieve a value in a field from the previous or next record in a form by using code, follow these steps:


  1. Create a new module and type the following line in the Declarations section if it is not already there:
    Option Explicit
  2. Type the following procedures:


    NOTE: In the following sample code, an underscore (_) at the end of a line is used as a line-continuation character. If you are using a version of Microsoft Access earlier than 7.0, remove the underscore from the end of the line when re-creating this code.
     '*************************************************************
    ' FUNCTION: PrevRecVal()
    ' PURPOSE: Retrieve a value from a field in the previous form
    ' record.
    ' PARAMETERS:
    ' F - The form from which to get the previous value.
    ' KeyName - The name of the form's unique key field.
    ' KeyValue - The current record's key value.
    ' FieldNameToGet - The name of the field in the previous
    ' record from which to retrieve the value.
    ' RETURNS: The value in the field FieldNameToGet from the
    ' previous form record.
    ' EXAMPLE:
    ' =PrevRecVal(Form,"ID",[ID],"OdometerReading")
    '**************************************************************
    Function PrevRecVal (F As Form, KeyName As String, KeyValue, _
    FieldNameToGet As String)
    Dim RS As Recordset

    On Error GoTo Err_PrevRecVal

    ' The default value is zero.
    PrevRecVal = 0

    ' Get the form recordset.
    Set RS = F.RecordsetClone

    ' Find the current record.
    Select Case RS.Fields(KeyName).Type
    ' Find using numeric data type key value?
    Case DB_INTEGER, DB_LONG, DB_CURRENCY, DB_SINGLE, _
    DB_DOUBLE, DB_BYTE
    RS.FindFirst "[" & KeyName & "] = " & KeyValue
    ' Find using date data type key value?
    Case DB_DATE
    RS.FindFirst "[" & KeyName & "] = #" & KeyValue & "#"
    ' Find using text data type key value?
    Case DB_TEXT
    RS.FindFirst "[" & KeyName & "] = '" & KeyValue & "'"
    Case Else
    MsgBox "ERROR: Invalid key field data type!"
    Exit Function
    End Select

    ' Move to the previous record.
    RS.MovePrevious

    ' Return the result.
    PrevRecVal = RS(FieldNameToGet)

    Bye_PrevRecVal:
    Exit Function
    Err_PrevRecVal:
    Resume Bye_PrevRecVal
    End Function

    '*************************************************************
    ' FUNCTION: NextRecVal()
    ' PURPOSE: Retrieve a value from a field in the next form
    ' record.
    '**************************************************************
    Function NextRecVal (F As Form, KeyName As String, KeyValue, _
    FieldNameToGet As String)
    Dim RS As Recordset

    On Error GoTo Err_NextRecVal

    ' The default value is zero.
    NextRecVal = 0

    ' Get the form recordset.
    Set RS = F.RecordsetClone

    ' Find the current record.
    Select Case RS.Fields(KeyName).Type
    ' Find using numeric data type key value?
    Case DB_INTEGER, DB_LONG, DB_CURRENCY, DB_SINGLE, _
    DB_DOUBLE, DB_BYTE
    RS.FindFirst "[" & KeyName & "] = " & KeyValue
    ' Find using date data type key value?
    Case DB_DATE
    RS.FindFirst "[" & KeyName & "] = #" & KeyValue & "#"
    ' Find using text data type key value?
    Case DB_TEXT
    RS.FindFirst "[" & KeyName & "] = '" & KeyValue & "'"
    Case Else
    MsgBox "ERROR: Invalid key field data type!"
    Exit Function
    End Select

    ' Move to the next record.
    RS.MoveNext

    ' Return the result.
    NextRecVal = RS(FieldNameToGet)

    Bye_NextRecVal:
    Exit Function
    Err_NextRecVal:
    Resume Bye_NextRecVal
    End Function
The following example demonstrates how to use the PrevRecVal() function in a form to create a mileage log. The custom function returns the previous odometer reading, which is used to calculate miles per gallon (MPG) for an automobile. To use the PrevRecVal() function, follow these steps:


  1. Create the following new table, and then save it as Mileage Log:

    Table: Mileage Log
    -----------------------------------------------------------------
    Field Name: ID
    Data Type : AutoNumber (or Counter in Access 2.0 or earlier)
    Indexed: Yes (No Duplicates)
    Field Name: Date
    Data Type : Date/Time
    Field Name: Odometer
    Data Type : Number
    FieldSize: Double
    Field Name: Gallons
    Data Type : Number
    FieldSize: Double
    Primary Key: ID
  2. View the Mileage Log table in Datasheet view and enter the following sample data:

    ID Date Odometer Gallons
    ---------------------------------
    1 6/21/94 77917.8 10.2
    2 6/25/94 78254.7 9.6
    3 6/30/94 78582.3 10
    4 7/5/94 78918.4 10.4
    5 7/10/94 79223.4 9.4
  3. Use the Form Wizard to create a new tabular form based on the Mileage Log table. Include all the Mileage Log table fields, except the ID field.
  4. View the form in Design view and add the following three text box controls to the form:

    Text Box 1
    -----------------------------------------------------
    Name: PrevOdometer
    ControlSource: =PrevRecVal(Form,"ID",[ID],"Odometer")
    Format: Fixed

    Text Box 2
    -----------------------------------------------------------------
    Name: MilesDriven
    ControlSource: =iif([PrevOdometer]=0,0,[Odometer]-[PrevOdometer])
    Format: Fixed

    Text Box 3
    ---------------------------------------
    Name: MPG
    ControlSource: =[MilesDriven]/[Gallons]
    Format: Fixed
  5. View the form in Form view. Note that the form displays the following information:

    Date Odometer Gallons PrevOdometer MilesDriven MPG
    -------------------------------------------------------------
    6/21/94 77917.8 10.2 0 0 0
    6/25/94 78254.7 9.6 77917.80 336.90 35.09
    6/30/94 78582.3 10.0 78254.70 327.60 32.76
    7/05/94 78918.4 10.4 78582.30 336.10 32.32
    7/10/94 79223.4 9.4 78918.40 305.00 32.45

References

For information about entering data automatically into a form by using values from the previous record, please see the following article in the Microsoft Knowledge Base:
136127 ACC: Fill Record w/Data from Prev. Record Automatically (95/97)
Lastnosti

ID članka: 101081 – Zadnji pregled: 18. jan. 2007 – Revizija: 1

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