i need help in my query..

i need to compute for the difference between two time.. i already have a formula for that but the problem is, i am setting up a condition before i get the difference between two diffrenet time...

i have a table named test.. it has 3 columns.. first column says start time, 2nd says end time.. columns 1 and 2 contains the time in hour:minutes:seconds.... the third one is about event id... this event id contains various number codings... the condition is if my event id is equal to 3, i must find another event id which is equal to 5... then i'll get the start time of event id 3 subtract it with the end time of event id 5 to get the time difference in seconds regardless how big the time may be... still it must be displayed in seconds..

i need a condition for getting their time difference with that condition.. i need the result to be displayed on my jsp so that with that query, i won't find any loading time issues.. thanks...