CEP is a processing engine that processes events in real-time.. To process events, events needs to send to CEP in some manner. In your case if any change occur db, there should be some other client or external party need send events to CEP. There are several default adaptors available which receive events. see the links [1] & [2] for more info..
[1] http://docs.wso2.org/display/CEP300/Input+Event+Adaptors
[2] http://wso2.com/library/articles/2013/08/writing-custom-event-adaptors-for-cep-3.0.0/