How To Cancel an Asynchronous Query--RDO

MORE INFORMATION

Sample Program

This example assumes that you already have an ODBC database server available and that you have an SQL query that runs long enough to make a cancel operation practical. This example uses a "DSN-less" ODBC connection, so you do not need to set up a DSN with the ODBC Admin utility.

1. Start a new project in Visual Basic. Form1 is created by default.
2. Add two command buttons to Form1. Change the caption property of Command1 to Begin and the caption property of Commmand2 to Cancel.
3. Paste the following code into the General Declarations section of form1:

         Option Explicit
         Dim cn As rdoConnection
         Dim en As rdoEnvironment
         Dim rs As rdoResultset
         Private Sub Form_Load()
           Command1.Enabled = True
           Command2.Enabled = False
           'Establish connection.
           Set en = rdoEngine.rdoEnvironments(0)
           en.CursorDriver = rdUseOdbc
           'This should be modified to connect to your database.
           Dim cnStr As String
           cnStr = "driver={SQL Server};server=mysvr;" & _
             "database=pubs;uid=myuid;pwd=mypwd"
           Set cn = en.OpenConnection(dsname:="", Prompt:=rdDriverNoPrompt, _
              Connect:=cnStr)
         End Sub
   
         Private Sub Command1_Click()
           Dim sql As String
           'Change this to an SQL statement that takes at least a few
           'seconds to finish, such as a Cartesian join or a text
           'column search, i.e. Where textcolumn Like '*find this text*'.
           sql = "Select title From Titles"
           Command1.Enabled = False
           Command2.Enabled = True
           Set rs = cn.OpenResultset(Name:=sql, Type:=rdOpenStatic, _
             Option:=rdAsyncEnable)
           While rs.StillExecuting
             DoEvents  ' Allow user to click on Cancel command button.
           Wend
           Command1.Enabled = True
           Command2.Enabled = False
         End Sub
   
         Private Sub Command2_Click()
           rs.Cancel
         End Sub
   
   						

4. Note that you need to change your DRIVER, SERVER, DATABASE, UID, and PWD in the OpenConnection function contained in the Form_Load event. You also need to modify the SQL statement contained in the Command1_Click event to match your own SQL data source and to be complex enough to make this example a practical use of the cancel operation. A good example is a Cartesian join or a text column search:

         Select * from mytable Where textcolumn Like '*find this text*'
   
   						

5. Start the program or press the F5 key.
6. You can then click the Begin Command button to start the query and the Cancel Command button to cancel the currently running query while it is in operation.