HOWTO: Display/Pass TimeStamp Value from/to SQL Server

• Microsoft Visual Basic Enterprise Edition for Windows, version 5.0

SUMMARY

TimeStamp is a SQL Server data type that is automatically updated every time a row is inserted or updated. Values in TimeStamp columns are not datetime data; they are, by default, defined as binary(8) varbinary(8), indicating the sequence of Microsoft SQL Server activity on the row. A table can have only one TimeStamp column. The TimeStamp data type is simply a monotonically-increasing counter whose values will always be unique within a database.

In Visual Basic, by opening an RDO cursor and setting the concurrency control options as the Locktype argument of the OpenResultset method, RDO will handle the locking automatically. However, there are situations in which you might prefer to perform all operations against the database with stored procedures at the server side. If this is the case, a TimeStamp value could be used to check if the row has been modified in between the time that it was first selected and the time that the update is posted.

This article provides the Visual Basic code that converts and displays the value in a string-type format to retrieve a TimeStamp column from a SQL Server table. Additionally, for comparison, the value is passed back to a stored procedure, which does the update based on the a WHERE clause evaluation of true. An error is raised if the concurrency check fails.

Keep in mind that TimeStamps were intended for use by SQL Server internally and, as such, the format is subject to change without notice.

MORE INFORMATION

The following example demonstrates how the value of a TimeStamp column is converted and displayed using a Visual Basic ListBox control. Additionally, it is then passed back to a stored procedure for concurrency control. The stored procedure, UpdateBalance, updates the table with new balance if the TimeStamp comparison is true; if the concurrency check fails, it raises an error and returns the error message back to Visual Basic. Function TSToHex() converts the TimeStamp column from data type binary to string.

Note that TimeStamps are opaque values that do not mean anything in and of themselves (like bookmarks). Using function TSToHex() and displaying the TimeStamp values on a list in this example is only for demonstration purposes. In real-world scenarios, the user never really needs to see or know about it.

Task One: Create Test Table and Stored Procedure

1. For testing purpose, table BankTbl is created in Pubs database with three rows. To create a table and add new rows in SQL Server, select the Pubs database, place the following Create Table T-SQL in the SQL window of ISQL/W and execute it:

         CREATE TABLE BankTbl
                (Account int identity, Balance money, Stamp TimeStamp)
         INSERT INTO BankTbl (Balance) Values(1000)
         INSERT INTO BankTbl (Balance) Values(400)
         INSERT INTO BankTbl (Balance) Values(250)
         CREATE UNIQUE INDEX AcctIndex ON BankTbl(Account)
   
   

2. To create the stored procedure updateBalance, place the following code in T-SQL in the SQL window of ISQL/W and execute it:

         CREATE PROCEDURE UpdateBalance
                @Sys_Ts TimeStamp, @newBalance Money AS
                UPDATE Pubs..BankTbl
                SET BankTbl.Balance = @newBalance
                WHERE BankTbl.Stamp=@Sys_Ts
                IF @@ROWCOUNT=0
                   RAISERROR ("Optimistic concurrency check failed!", 11, -1)
   
   
   

Task Two: Build the Visual Basic Code

1. Start a new project in Visual Basic and choose "Standard EXE." Form1 is created by default.

2. From the Project menu, select References, and then place a check next to Microsoft Remote Data Object 2.0.

3. Add a ListBox, a TextBox and a CommandButton to Form1.

4. Paste the following code in the General Declaration section of Form1:

         Option Explicit
         Dim cn As New rdoConnection
         Dim rsList As rdoResultset
   

         Function TSToHex(sBinRep As rdoColumn) As String
            Dim sBuffer As String
            Dim b As Integer
   
            sBuffer = "0x"
            For b = 1 To 8        'Break up the binary
            sBuffer = sBuffer + Right$("00" & _
                      Hex(AscB(MidB(sBinRep, b, 1))), 2)
            Next b
            TSToHex = sBuffer     'Return the string
         End Function
   
         Private Sub Command1_Click()
            Dim strTS As String
            Dim theBalance As Single
            Dim strTemp As String
            Dim oldPos As Integer
   
            On Error GoTo Err_Update
            If List1.ListIndex = -1 Then
               MsgBox "No item is selected"
               Exit Sub
            End If
   
            strTemp = Mid(List1.Text, InStr(List1.Text, Chr(9)) + 1)
            theBalance = Val(Mid(strTemp, 1, InStr(strTemp, Chr(9)) - 1)) _
                         - Val(Text1.Text)
            strTS = Mid(strTemp, InStr(1, strTemp, Chr(9)) + 1)
   
            'if the concurrency check fails, a runtime error will occur
            cn.Execute "{CALL UpdateBalance(" _
                      & strTS & ", " & theBalance & ")}"
   
            rsList.Requery
            oldPos = List1.ListIndex
            List1.Clear
            Do While Not rsList.EOF
               List1.AddItem rsList(0) & Chr(9) & rsList(1) & Chr(9) _
                                       & TSToHex(rsList(2))
               rsList.MoveNext
            Loop
            List1.ListIndex = oldPos
            MsgBox "Update Succeeded", vbOKOnly
            Exit Sub
         Err_Update:
            MsgBox Err.Description, vbCritical
         End Sub
   
         Private Sub Form_Load()
            Dim strSQL As String
            Dim strConnect As String
   
            strConnect = "Driver={SQL Server};Server=MyServer; _
                          Database=pubs;Uid=sa;Pwd="
            With cn
               .Connect = strConnect
               .EstablishConnection
            End With
            strSQL = "Select Account, Balance, Stamp from BankTbl _
                      Order by Account"
            Set rsList = cn.OpenResultset(strSQL)
            Do While Not rsList.EOF
               List1.AddItem rsList(0) & Chr(9) & rsList(1) & Chr(9) _
                                       & TSToHex(rsList(2))
               rsList.MoveNext
            Loop
            List1.ListIndex = 0
            Text1.Text = "0"
            Text1.TabIndex = 0
            Command1.Default = True
         End Sub
   
         Private Sub Form_Unload(Cancel As Integer)
            cn.Close
         End Sub
   
   

5. Note that you should change your SERVER, UID, and PWD parameters in the RDO connection string.

6. Start the program or press the F5 key.

7. Withdraw $50 dollars from account number 2 by selecting the second item in the ListBox, entering 50 into the TextBox, and then clicking Command1 to make the transaction.