SQL Window function to get 2 closest events - sql

Question

I was trying to solve the "analyze weather patterns" problem as described here (https://joins-238123.netlify.com/window-functions/)
You're worried that hurricanes are happening more frequently, so you
decide to do a tiny bit of analysis. For each kind of weather event
find the 2 events that occurred the closest together and when they
happened
Table weather with data like:
type day
rain 6
rain 12
thunderstorm 13
rain 21
rain 27
rain 37
rain 44
rain 54
thunderstorm 56
rain 58
rain 61
rain 65
rain 68
rain 73
rain 82
hurricane 87
rain 92
rain 95
rain 98
rain 108
thunderstorm 111
rain 118
rain 123
rain 128
rain 131
hurricane 135
rain 136
rain 140
rain 149
thunderstorm 158
rain 159
rain 167
rain 175
hurricane 178
rain 179
rain 186
rain 192
rain 200
thunderstorm 202
rain 210
rain 219
thunderstorm 222
rain 226
rain 232
thunderstorm 238
rain 241
rain 246
rain 253
thunderstorm 257
rain 257
rain 267
rain 277
rain 286
rain 295
rain 302
rain 307
thunderstorm 312
rain 316
rain 325
thunderstorm 330
I could come up with :
select type, day, COALESCE(day - LAG(day, 1) over (partition by type order by day), 0) as days_since_previous from weather
It gives me results like:
type day days_since_previous
hurricane 87 0
hurricane 135 48
hurricane 178 43
rain 6 0
rain 12 6
rain 21 9
rain 27 6
But I can't get it to narrow the results down to the 2 closest events and only display the days between them.
How do I go about doing so that I get the desired result like:
type day days_since_previous
rain 61 3
hurricane 178 43
thunderstorm 238 16

Answer 1

You can use another window function to widdle down the rows:
SELECT type, day, days_since_previous
FROM (
SELECT type, day, (day - prev_day) AS days_since_previous,
ROW_NUMBER() OVER(PARTITION BY type ORDER BY (day - prev_day)) AS RowNum
FROM (
select type, day,
LAG(day, 1) over (partition by type order by day) as prev_day
from weather
) src
WHERE prev_day IS NOT NULL -- Ignore "first" events
) src
WHERE RowNum = 1
order by day
I also removed the COALESCE since that was causing the "first" events to be included in the calculations.

Answer 2

If you don't insist on displaying the day value - you could run a nested query:
In one SELECT (in a WITH clause, or a nested sub-select) add the gap to previous day as an OLAP function, as you suggest. No need to COALESCE, really ..
From that fullselect , run a GROUP BY select.
Like so:
WITH
w_gap2prev AS (
SELECT
*
, day - LAG(day) OVER(PARTITION BY type ORDER BY day) AS gap
FROM input
)
SELECT
type
, MIN(gap) AS days_since_previous
FROM w_gap2prev
WHERE gap IS NOT NULL
GROUP BY type
;
-- out type | days_since_previous
-- out --------------+---------------------
-- out hurricane | 43
-- out rain | 3
-- out thunderstorm | 16
-- out (3 rows)
-- out
-- out Time: First fetch (3 rows): 56.441 ms. All rows formatted: 56.479 ms