SQLite: create equal date ranges and query data based on them? - sqlite

Question

I have datas in a table with schema:
Id INTEGER,
date DATETIME,
value REAL
id is primary key, and I have an index on date column to speed up querying values within a specific date range.
What should I do if I need N equal date ranges between specific start and end dates, and query aggregated datas for each date range?
For example:
Start date: 2015-01-01
End date: 2019-12-31
N: 5
In this case equal date intervals should be:
2015-01-01 ~ 2015-12-31
2016-01-01 ~ 2016-12-31
2017-01-01 ~ 2017-12-31
2018-01-01 ~ 2018-12-31
2019-01-01 ~ 2019-12-31
And the query should aggregate all values (AVG) in between those intervals, so I would like to have 5 total rows after the execution.
Maybe something with CTE?

Answer 1

There are 2 ways to do it.
They both use recursive ctes but return different results.
The 1st one with NTILE():
with
dates as (select '2015-01-01' mindate, '2019-12-31' maxdate),
alldates as (
select mindate date from dates
union all
select date(a.date, '1 day')
from alldates a cross join dates d
where a.date < d.maxdate
),
groups as (
select *, ntile(5) over (order by date) grp
from alldates
),
cte as (
select min(date) date1, max(date) date2
from groups
group by grp
)
select * from cte;
Results:
| date1 | date2 |
| ---------- | ---------- |
| 2015-01-01 | 2016-01-01 |
| 2016-01-02 | 2016-12-31 |
| 2017-01-01 | 2017-12-31 |
| 2018-01-01 | 2018-12-31 |
| 2019-01-01 | 2019-12-31 |
And the 2nd builds the groups with math:
with
dates as (select '2015-01-01' mindate, '2019-12-31' maxdate),
cte1 as (
select mindate date from dates
union all
select date(
c.date,
((strftime('%s', d.maxdate) - strftime('%s', d.mindate)) / 5) || ' second'
)
from cte1 c inner join dates d
on c.date < d.maxdate
),
cte2 as (
select date date1, lead(date) over (order by date) date2
from cte1
),
cte as (
select date1,
case
when date2 = (select maxdate from dates) then date2
else date(date2, '-1 day')
end date2
from cte2
where date2 is not null
)
select * from cte
Results:
| date1 | date2 |
| ---------- | ---------- |
| 2015-01-01 | 2015-12-31 |
| 2016-01-01 | 2016-12-30 |
| 2016-12-31 | 2017-12-30 |
| 2017-12-31 | 2018-12-30 |
| 2018-12-31 | 2019-12-31 |
In both cases you can get the averages by joining the table to the cte:
select c.date1, c.date2, avg(t.value) avg_value
from cte c inner join tablename t
on t.date between c.date1 and c.date2
group by c.date1, c.date2