Vertica/SQL: Getting rows immediately proceeding events - sql

Question

Consider a simple query
select from tbl where status=MELTDOWN
I would like to now create a table that in addition to including these rows, also includes the previous p rows and the subsequent n rows, so that I can get a sense as to what happens in the surrounding time of these MELTDOWNs. Any hints?

Answer 1

You can do this with window functions by getting the seqnum of the meltdown rows. I prefer to do this with lag()/lead() ignore nulls, but Vertical doesn't support that. I think this is the equivalent with first_value()/last_value():
with t as (
select t.*, row_number() over (order by id) as seqnum
from tbl
),
tt as (
select t.*,
last_value(case when status = 'meltdown' then seqnum end ignore nulls) over (order by seqnum rows between unbounded preceding and current row) as prev_meltdown_seqnum,
first_value(case when status = 'meltdown' then seqnum end ignore nulls) over (order by seqnum rows between current row and unbounded following) as prev_meltdown_seqnum,
from t
)
select tt.*
from tt
where seqnum between prev_melt_seqnum and prev_melt_seqnum + 7 or
seqnum between next_melt_seqnum -5 and next_melt_seqnum;

Answer 2

WITH
grouped AS
(
SELECT
SUM(
CASE WHEN status = 'Meltdown' THEN 1 ELSE 0 END
)
OVER (
ORDER BY timeStamp
)
AS GroupID,
tbl.*
FROM
tbl
),
sorted AS
(
SELECT
ROW_NUMBER() OVER (PARTITION BY GroupID ORDER BY timeStamp ASC ) AS incPos,
ROW_NUMBER() OVER (PARTITION BY GroupID ORDER BY timeStamp DESC) AS decPos,
MAX(GroupID) OVER () AS LastGroup
grouped.*
FROM
grouped
)
SELECT
sorted.*
FROM
sorted
WHERE
(incPos <= 8 AND GroupID > 0 ) -- Meltdown and the 7 events following it
OR (decPos <= 6 AND GroupID <> LastGroup) -- and the 6 events preceding a Meltdown
ORDER BY
timeStamp