Open the trigger named invoices_before_update that was shown in figure 16-2. Then, modify it so it also raises an error whenever the payment total plus the credit total becomes larger than the invoice total in a row. Then, test this trigger with an appropriate UPDATE statement. Create a trigger named invoices_after_update. This trigger should insert the old data about the invoice into the Invoices_Audit table after the row is updated. Then, test this trigger with an appropriate UPDATE statement. If the Invoices_Audit table doesn't exist, you can use the code shown in figure 16-3 to create it. Check whether the event scheduler is turned on. If it isn't, code a statement that turns it on. Then, create an event that inserts a test row that contains test values into the Invoices_Audit table every minute. To make sure that this event has been created, code a SHOW EVENTS statement that views this event and a SELECT statement that views the data that's inserted into the Invoices Audit table. Once you're sure this event is working correctly, code a DROP EVENT statement that drops the event.