select id = max(id), topic, category, date = max(date)
  into #tmp
  from alarms
 group by topic, category

select t1.topic, t1.category, t1.date,
       t2.category, t2.date
  from #tmp t1,
       #tmp t2
 where t1.category = "WARNING"
   and t2.category = "OK NOW"
   and t1.topic = t2.topic
   and t1.date < t2.date
UNION
select t1.topic, t1.category, t1.date,
       NULL, NULL
  from #tmp t1,
       #tmp t2
 where t1.category = "WARNING"
   and t2.category = "OK NOW"
   and t1.topic = t2.topic
   and t1.date > t2.date