ACC2000: How to Fill a Table with Random Records from Another Table (210616)

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One way that you can get a certain number of random records from a table is to create a query based on the table. You can create an expression that applies the Rnd() function to a Number field in the table; if there is no Number field available, you can apply the Len() function to a text field and then apply the Rnd() function to the result. Next, you set the sort order of this calculated field to ascending or descending, and then select from the table the other fields that you want to see in the query result. Finally, you set the query's TopValues property to the number of records that you want.

However, there is a disadvantage to this method. When you start Access and run the query, a particular set of records is returned, which can be referred to as "Recordset A." You may run the query several more times with different results, and then quit Access. The next time you start Access and run the query, the result is again Recordset A.

One resolution is to run a procedure that uses DAO to fill a table with a specified number of records from an existing table; this procedure also uses the Randomize statement to reinitialize the random-number generator. The following steps demonstrate how to use the sample subroutine BuildRandomTable.

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.

1. Open the sample database Northwind.mdb.
2. Create the following table:

       Table: tblRandom
       -------------------------------
       Field Name: lngGuessNumber
       Data Type: Number
       Field Size: Long Integer
       Indexed: Yes (No Duplicates)
   
       Field Name: lngOrderNumber
       Data Type: Number
       Field Size: Long Integer
       Indexed: No
   
       Table Properties: tblRandom
       ---------------------------
       PrimaryKey: lngGuessNumber
   					

3. Create a module and type the following line in the Declarations section if it is not already there:

    Option Explicit
   					

4. Type the following procedure:

    Sub BuildRandomTable(lngRequest as Long)
   
      Dim dbsRandom As Database
      Dim rstOrders As Recordset
      Dim rstRandom As Recordset
      Dim UpperLimit As Long
      Dim LowerLimit As Long
      Dim lngCounter As Long
      Dim lngGuess As Long
      Dim lngRecordCount As Long
   
      ' Assumes that this module is in the Northwind database.
      Set dbsRandom = CurrentDb
      ' Delete any existing records from tblRandom table.
      dbsRandom.Execute "Delete * from tblRandom;"
      ' Open Orders as a Table Type recordset.
      Set rstOrders = dbsRandom.OpenRecordset("Orders", dbOpenTable)
      rstOrders.MoveFirst
      LowerLimit = rstOrders!OrderID
      rstOrders.MoveLast
      UpperLimit = rstOrders!OrderID
      lngRecordCount = rstOrders.RecordCount
   
      Set rstRandom = dbsRandom.OpenRecordset("tblRandom", _
                          dbOpenDynaset)
      lngCounter = 1
   
      ' Check to make sure the number of
      ' records requested is reasonable.
      If lngRequest > lngRecordCount Then
         MsgBox "Request is greater than the total number of records."
         Exit Sub
      Else
         lngRequest = lngRequest + 1
      End If
   
      Randomize
      Do Until lngCounter = lngRequest
         ' Generate a random number
         lngGuess = Int((UpperLimit - LowerLimit + 1) * Rnd + LowerLimit)
         ' Ensure that it exists in the Orders table.
         rstOrders.Index = "PrimaryKey"
         rstOrders.Seek "=", lngGuess
         If rstOrders.NoMatch Then
            ' Drop through and generate a new number.
         Else
            ' Check to see if it's already been used in the new table.
            rstRandom.FindFirst "lngOrderNumber =" & lngGuess
            ' If not, add it to the new table.
            If rstRandom.NoMatch Then
               With rstRandom
                  .AddNew
                  !lngGuessNumber = lngCounter
                  !lngOrderNumber = lngGuess
                  .Update
               End With
                  lngCounter = lngCounter + 1
            End If
         End If
      Loop
      ' Clean up.
      dbsRandom.Close
   
    End Sub
   					

5. To test this procedure, type the following line in the Immediate window, and then press ENTER.

    BuildRandomTable 5