How to query SQLite data hourly? - sqlite

Question

The data table looks like the following:
ID DATE
1 2020-12-31 10:10:00
2 2020-12-31 20:30:00
3 2020-12-31 20:50:00
4 2021-01-02 17:10:00
5 2021-01-02 17:20:00
6 2021-01-02 17:30:00
7 2021-01-03 23:10:00
..
And I would like to query only the last entry per hour per day, and to have the resulte like:
ID DATE
1 2020-12-31 10:10:00
3 2020-12-31 20:50:00
6 2021-01-02 17:30:00
7 2021-01-03 23:10:00
..
I tried to look for hourly query and found the following
strftime('%H', " + DATE + ", '+1 hours')
However, not sure how to use it properly (e.g. with GROUP BY ? then how to ensure it takes the lastest entry of the hour), therefore, would be great to have some help here!

Answer 1

You can do it with ROW_NUMBER() window function:
SELECT ID, DATE
FROM (
SELECT *,
ROW_NUMBER() OVER (PARTITION BY strftime('%Y%m%d%H', DATE) ORDER BY DATE DESC) rn
FROM tablename
)
WHERE rn = 1
ORDER BY ID
Instead of strftime('%Y%m%d%H', DATE) you could also use substr(DATE, 1, 13).
For versions of SQLite previous to 3.25.0 which do not support window functions you can do it with NOT EXISTS:
SELECT t1.*
FROM tablename t1
WHERE NOT EXISTS (
SELECT 1
FROM tablename t2
WHERE strftime('%Y%m%d%H', t2.DATE) = strftime('%Y%m%d%H', t1.DATE)
AND t2.DATE > t1.DATE
)
See the demo.
Results:
> ID | DATE
> -: | :------------------
> 1 | 2020-12-31 10:10:00
> 3 | 2020-12-31 20:50:00
> 6 | 2021-01-02 17:30:00
> 7 | 2021-01-03 23:10:00