The REGISTER EVENT statement tells the server that you would like to be notified of all future occurrences of the specified event, even if you are not yet waiting forit. By separating the REGISTER EVENT and WAIT EVENT commands, you can start queuing events immediately, while waiting until later to actually start processing them.
You do not need to register for every event before waiting for it. When you wait on an event, you will be registered implicitly for that event if you did not already explicitly register for it. Thus you only need to explicitly register events if you want them to start being queued now but you do not want to start waiting for them until later.
The UNREGISTER EVENT statement unregisters the specified event.
The REGISTER EVENT and UNREGISTER EVENT statement are allowed only inside stored procedures.