if в запросе

sum(iif(e>5, e, 0))

Case
when поле1:>переменная then 
поле2
else 0
end псевдоним

select sum(extract(second from LOG.DURATION )) ITOG, 
Case
when LOG.DURATION :>'00:05:00' then 
sum(extract(second from LOG.DURATION))
else 0
end ITOG2,
EXT_LINE.EXT, EXT_LINE.NAME
from LOG, EXT_LINE
where LOG.ID_EXT_LINE=EXT_LINE.ID_EXT_LINE
group by EXT_LINE.EXT, EXT_LINE.NAME
order by EXT_LINE.EXT, EXT_LINE.NAME