how do i implement this thing

select id,date,pact
from case

if the below conditions gives a matching record then don,t go to the 2nd condition
1)where pact='abcd'
and date='11-oct-2006'
if the 1) condition doesn,t give any record then implement this below one

2)[COLOR=MediumTurquoise]where pact='abcd'
and date='16-oct-2006'

it is kind of if else analogy to be implemented on select query

thanks in advance