clickhouse runningDifference inside groups

runningDifference_group_by.sql

DROP TABLE IF EXISTS test;

CREATE TABLE test (sessionid String, timestamp DateTime) ENGINE = Log;

INSERT INTO test SELECT 's1', arrayJoin([now(), now() + 10, now() + 21, now() + 33]);
INSERT INTO test SELECT 's2', arrayJoin([now()+33, now() + 34, now() + 35, now() + 36]);
INSERT INTO test SELECT 's3', arrayJoin([now()+40, now() + 60, now() + 61, now() + 80]);

WITH
  groupArray( timestamp ) as all_timestamps_in_group,
  arrayEnumerate( all_timestamps_in_group ) as all_timestamps_in_group_ids,
  arrayMap( i -> i > 1 ? all_timestamps_in_group[i] - all_timestamps_in_group[i - 1] : 0, all_timestamps_in_group_ids) as timestamp_diff
SELECT 
  sessionid,
  timestamp_diff
FROM test
GROUP BY sessionid;