Group a sequence of lines [SQL] - mysql

Question

Is there a way to group a sequence of rows in SQL (MySQL 5.1.73).
Let me explain, I have a query that gives this:
hour
start_date
end_date
10
2022-02-01 10:11:18
2022-02-01 10:50:18
11
2022-02-01 11:30:31
2022-02-01 11:38:12
13
2022-02-17 13:55:09
2022-02-17 13:58:38
14
2022-02-17 14:51:09
2022-02-17 14:57:59
And I would like to convert it to this:
hour
start_date
end_date
10
2022-02-01 10:11:18
2022-02-01 11:38:12
13
2022-02-17 13:55:09
2022-02-17 14:57:59
Indeed, I would like to group all the lines whose hours follow each other.
My request is a grouping in hours, like this :
SELECT hour( date ) as hour, MIN(date) as start_date , MAX(date) as end_date
FROM test_tbl
GROUP BY hour( date ) , date( date )
order by date, hour( date ) ;
But after doing this query, I would like to group the lines whose hours follow each other (10,11 => 10)...

Answer 1

EDIT: the following answer only works with MySQL version 8+
with tbl_by_hour as (
SELECT hour( date ) as hour, MIN(date) as start_date , MAX(date) as end_date
FROM test_tbl
GROUP BY hour( date ) , date( date )
order by date, hour( date )
)
select
min(hour) as hour,
min(start_date) as start_date,
max(end_date) as end_date
from (
select tab1.*,
sum(case when prev_hour is null or prev_hour = hour - 1 then 0 else 1 end) over(order by hour) grp
from (
select hour, start_date, end_date, lag(hour) over(order by hour) prev_hour from tbl_by_hour
) as tab1
) as tab2
group by grp

Answer 2

You can probably do something like this:
SELECT MIN(hours), dates, MIN(start_date), MAX(end_date), tn
FROM
(SELECT *,
CEIL(rownum/5) AS tn
FROM
(SELECT *,
CASE WHEN dates=#dt
AND hours=#hr+1
THEN #rn := #rn+1
WHEN dates=#dt
AND hours > #hr+1
THEN #rn := #rn+20
ELSE #rn := 1
END AS rownum,
#dt := dates,
#hr := hours
FROM
(SELECT hour(date) as hours, date(date) dates,
MIN(date) as start_date , MAX(date) as end_date
FROM test_tbl t
GROUP BY dates, hours) v
CROSS JOIN (SELECT #rn := 0, #dt := NULL, #hr := 0) r
ORDER BY dates, hours) s
) w
GROUP BY dates, tn;
I took your original query as base then made it as subquery.
Then I CROSS JOIN with a subquery of variables where I'm attempting to generate a custom row numbering. The conditions of the row number are:
If it's on the same date and the next hour increment from previous is +1 then continue the numbering.
If it's on the same date and the next hour increment from previous more than +1 then pick-up the last number and increment it by +20.
Repeat the row numbering sequence if the date is different.
After generating the row numbering, I convert to subquery then divide the row numbering by 5 and use ceiling (CIEL) function to somehow make them the same, effectively identifying (assuming) these rows with same CIEL(rownum/5) result as one group - this is where I felt it's not really convincing but it works anyhow.
Lastly, I convert that to a subquery again and did the whole MIN(hours), dates, MIN(start_date), MAX(end_date), tn with GROUP BY dates, tn.
It's not a convincing solution because the final operation (generating of the tn column) is based on creativity and not something certain. I usually prefer a solution that covers all the possible scenarios with something concrete rather than creative. However, I did some extensive tests on the current query with more data variation and so far it's returning good results. Also, I do notice that you said your MySQL version is 5.1+ so, I'm not really sure if this particular operation will work. Version 5.5+ is probably the lowest version of MySQL fiddle that is available online.
Here's a demo fiddle