Database Update And Insert Problem

Hi,
I have 3 short(ish) questions, If someone could help I'd be very grateful..
This is the situation - I have a formview which contains a button, which on clicking should insert a row into one table, and update a column value of another. I will provide the code at the end.
 
1. On clicking, I get the error:
  System.Data.SqlClient.SqlException: Incorrect syntax near the keyword 'WHERE'.
it doesnt say where, but the stack trace is:
[SqlException (0x80131904): Incorrect syntax near the keyword 'WHERE'.]
System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection) +180
System.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection) +68
System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj) +199
System.Data.SqlClient.TdsParser.Run(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj) +2411
System.Data.SqlClient.SqlCommand.FinishExecuteReader(SqlDataReader ds, RunBehavior runBehavior, String resetOptionsString) +147
System.Data.SqlClient.SqlCommand.RunExecuteReaderTds(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, Boolean async) +1089
System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, DbAsyncResult result) +314
System.Data.SqlClient.SqlCommand.InternalExecuteNonQuery(DbAsyncResult result, String methodName, Boolean sendToPipe) +413
System.Data.SqlClient.SqlCommand.ExecuteNonQuery() +115
System.Web.UI.WebControls.SqlDataSourceView.ExecuteDbCommand(DbCommand command, DataSourceOperation operation) +395
System.Web.UI.WebControls.SqlDataSourceView.ExecuteUpdate(IDictionary keys, IDictionary values, IDictionary oldValues) +643
System.Web.UI.DataSourceView.Update(IDictionary keys, IDictionary values, IDictionary oldValues, DataSourceViewOperationCallback callback) +78
System.Web.UI.WebControls.FormView.HandleUpdate(String commandArg, Boolean causesValidation) +1151
System.Web.UI.WebControls.FormView.HandleEvent(EventArgs e, Boolean causesValidation, String validationGroup) +429
System.Web.UI.WebControls.FormView.OnBubbleEvent(Object source, EventArgs e) +88
System.Web.UI.Control.RaiseBubbleEvent(Object source, EventArgs args) +35
System.Web.UI.WebControls.FormViewRow.OnBubbleEvent(Object source, EventArgs e) +109
System.Web.UI.Control.RaiseBubbleEvent(Object source, EventArgs args) +35
System.Web.UI.WebControls.Button.OnCommand(CommandEventArgs e) +86
System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) +155
System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +7
System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +11
System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +33
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +4886 
 
 
 
2. When updating, at the moment it will update with a new quantity provided in the text box, but id like the quantity in the textbox subtracted from the current column value - how would this be done?
 
3. For inserting, i was wondering if i need a command in the button_click to activate the insert command, similar to the one i have one for the update. I have tried but i cant seem to get the syntax right...
 
 
Code: 
 
 private bool ExecuteUpdate(int quantity)
{
  SqlConnection con = new SqlConnection();
  con.ConnectionString = "Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\ASPNETDB.MDF;Integrated Security=True;User Instance=True";


  con.Open();

  SqlCommand command = new SqlCommand();
  command.Connection = con;
  TextBox TextBox1 = (TextBox)FormView1.FindControl("TextBox1");
  Label labname = (Label)FormView1.FindControl("Label3");
  Label labid = (Label)FormView1.FindControl("Label13");

  command.CommandText = "UPDATE Items SET Quantityavailable = @qty WHERE productID=@productID";
  command.Parameters.Add("@qty", TextBox1.Text);
  command.Parameters.Add("@productID", labid.Text);
 command.ExecuteNonQuery();

  con.Close();
  return true;
}


    private bool ExecuteInsert(String quantity)
    {
        SqlConnection con = new SqlConnection();
        con.ConnectionString = "Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\ASPNETDB.MDF;Integrated Security=True;User Instance=True";

        con.Open();

        SqlCommand command = new SqlCommand();
        command.Connection = con;
        TextBox TextBox1 = (TextBox)FormView1.FindControl("TextBox1");
        Label labname = (Label)FormView1.FindControl("Label3");
        Label labid = (Label)FormView1.FindControl("Label13");

        command.CommandText = "INSERT INTO Transactions (Usersname)VALUES (@User)"+
          "INSERT INTO Transactions (Itemid)VALUES (@productID)"+
          "INSERT INTO Transactions (itemname)VALUES (@Itemsname)"+
          "INSERT INTO Transactions (Date)VALUES (+DateTime.Now.ToString() +)"+
          "INSERT INTO Transactions (Qty)VALUES (@qty)"+
        command.Parameters.Add("@User", System.Web.HttpContext.Current.User.Identity.Name);
        command.Parameters.Add("@Itemsname", labname.Text);
        command.Parameters.Add("@productID", labid.Text);
        command.Parameters.Add("@qty", TextBox1.Text);
        command.ExecuteNonQuery();

        con.Close();
        return true;
    }





protected void Button2_Click(object sender, EventArgs e)
{
  TextBox TextBox1 = FormView1.FindControl("TextBox1") as TextBox;
  ExecuteUpdate(Int32.Parse(TextBox1.Text) );

}
 
 
Huge thanks for anyone who can help shed some light!! 
 
Thanks again,
Jon