Microsoft KB Archive/210504

= ACC2000: Referring to a Field in the Previous Record or Next Record =

Article ID: 210504

Article Last Modified on 6/23/2005

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APPLIES TO


 * Microsoft Access 2000 Standard Edition

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This article was previously published under Q210504



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

This article applies only to a Microsoft Access database (.mdb).



SUMMARY
This article describes 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: You can see a demonstration of the technique that is used in this article in the sample file Qrysmp00.exe. For information about how to obtain this sample file, please see the following article in the Microsoft Knowledge Base:

207626 ACC2000: Access 2000 Sample Queries Available in Download Center



Method 1: Using DLookup
NOTE: This technique assumes that you have a table with an ID field of a Number data type, and that the ID values are not missing any number (or numbers) in sequential order. If your table does not meet these criteria, 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.

NOTE: You must insert the correct data field, table, form, report, and ID field names in the following sample expressions.

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("[ ]"," ","[ ]=Forms![ ]![ ]-1")

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

=DLookUp("[ ]"," ","[ ]=Forms![ ]![ ]+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("[ ]"," ","[ ]=Reports![ ]![ ]-1")

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

=DLookUp("[ ]"," ","[ ]=Reports![ ]![ ]+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("[ ]"," ","[ ]=" & [ ]-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("[ ]"," ","[ ]=" & [ ]+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.

Method 2: Using Code
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. NOTE: The sample code in this article uses Microsoft Data Access Objects. For this code to run properly, you must reference the Microsoft DAO 3.6 Object Library. To do so, click References on the Tools menu in the Visual Basic Editor, and make sure that the Microsoft DAO 3.6 Object Library check box is selected.

To retrieve a value in a field from the previous or next record in a form by using code, follow these steps:  Create a new module and type the following line in the Declarations section if it is not already there:

Option Explicit

  Type the following procedures: '************************************************************* ' 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 DAO.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 DAO.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:  Create the following new table, and then save it as Mileage Log:

Field Name: ID

Data Type : AutoNumber

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

  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                     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.</li> 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

</li>  View the form in Form view. Note that the form displays the following information: <pre class="fixed_text">  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                    </li></ol>

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