Finding the difference between the latest and the second latest term

The structure of the table is like create table events( event_type integer not null, value integer not null, time timestamp not null, unique (event_type ,time) ); Have inserted few values like insert into events values (2, 5, '2015-05-09 12:42:00'), (4, -42, '2015-05-09 13:19:57'), (2, 2, '2015-05-09 14:48:39'), (2, 7, '2015-05-09 13:54:39'), (3, 16, '2015-05-09 13:19:57'), (3, 20, '2015-05-09 15:01:09') I want to write a query that for each event_type that has been registered more than once returns the difference between the latest and the second latest value . Given the above data, the output should be like event_type value 2 -5 3 4 I can bring out the lowest and the highest using below queries . This is how far I could reach ..confused on the way further . select event_type,value,time from events group by event_type order by event_type desc; select * from events group by event_type order by event_type ;