ถ้าบันทึกข้อมูลลูกค้าแล้วมันมีรหัสลูกค้าอยู่ใน access อยู่แล้ว ให้มันโชว์ messagebox ว่า รหัสลูกค้าซ้ำอ่ะค่ะ ใช้ฐานข้อมูล Access ภาษา VB2008 ค่ะ

    Sub btnSave_Click(sender As Object, e As EventArgs)        
			Dim intNumRows As Integer
			strSQL = "SELECT COUNT(*) FROM customer WHERE CustomerID = '"& Me.txtCustomerID.Text &"' "		
			objCmd = New OleDbCommand(strSQL, objConn)
			intNumRows = objCmd.ExecuteScalar()

			IF intNumRows > 0 Then
				Me.pnlAdd.Visible = False
				Me.lblStatus.Visible = True
				Me.lblStatus.Text = "CustomerID already exist."
			Else		
				strSQL = "INSERT INTO customer (CustomerID,Name,Email,CountryCode,Budget,Used) " & _
				" VALUES " & _
				" ('" & Me.txtCustomerID.Text & "','" & Me.txtName.Text & "','" & Me.txtEmail.Text & "', " & _
				" '" & Me.txtCountryCode.Text & "','" & Me.txtBudget.Text & "','" & Me.txtUsed.Text & "')"

				objCmd = New OleDbCommand
				With objCmd
				   .Connection = objConn
				   .CommandText = strSQL
				   .CommandType = CommandType.Text		   
				End With
								
				Me.pnlAdd.Visible = False
				Try
					objCmd.ExecuteNonQuery()            
					Me.lblStatus.Text = "Record Inserted"
					Me.lblStatus.Visible = True
				Catch ex As Exception
					Me.lblStatus.Visible = True
					Me.lblStatus.Text = "Record can not insert Error ("& ex.Message &")"
				End Try
		End IF

    End Sub