postgresql create table of days per month - sql

Question

I'm trying to write a script which returns a list of months with the number of days in the month. It references this table
CREATE TABLE generic.time_series_only (measurementdatetime TIMESTAMP WITHOUT TIME ZONE NOT NULL)
which is just a chronological time series (and very useful when joining tables of data with gaps in different places, but you want an unbroken timeseries as your output, maybe there's a smarter way to do that but I haven't found it yet).
SELECT date_part('year'::text, time_series_only.measurementdatetime) AS
measyear,
date_part('month'::text, time_series_only.measurementdatetime) AS
measmonth,
date_trunc('month'::text, time_series_only.measurementdatetime) +
'1 mon'::interval - date_trunc('month'::text,
time_series_only.measurementdatetime) AS days_in_month
FROM generic.time_series_only
GROUP BY date_part('year'::text, time_series_only.measurementdatetime),
date_part('month'::text, time_series_only.measurementdatetime)
ORDER BY date_part('year'::text, time_series_only.measurementdatetime),
date_part('month'::text, time_series_only.measurementdatetime);
But I get this error:
ERROR: column "time_series_only.measurementdatetime" must appear in the GROUP BY clause or be used in an aggregate function
I can't put this column in the GROUP BY clause because then I'd get a result for every single entry in the time_series_only table, and I can't figure a way to get the same result using an aggregate function? Any suggestions very welcome :-)

Answer 1

you not using generate_series?.. like here:
vao=# with pre as (select generate_series('2016-01-01','2017-03-31','1 day'::interval) g) select distinct
extract('year' from g), extract('month' from g), count(1) over (partition by date_trunc('month',g)) from pre order by 1,2;
date_part | date_part | count
-----------+-----------+-------
2016 | 1 | 31
2016 | 2 | 29
2016 | 3 | 31
2016 | 4 | 30
2016 | 5 | 31
2016 | 6 | 30
2016 | 7 | 31
2016 | 8 | 31
2016 | 9 | 30
2016 | 10 | 31
2016 | 11 | 30
2016 | 12 | 31
2017 | 1 | 31
2017 | 2 | 28
2017 | 3 | 31
(15 rows)

Answer 2

Use distinct on a pair (year, month). You can replace the time_series_only table with the function generate_series() , e.g.:
select distinct on (date_part('year', d), date_part('month', d))
date_part('year', d) as year,
date_part('month', d) as month,
date_part('day', d) as days_in_month
from
generate_series('2016-01-01'::date, '2016-12-31'::date, '1d'::interval) d
order by 1, 2, 3 desc;
year | month | days_in_month
------+-------+---------------
2016 | 1 | 31
2016 | 2 | 29
2016 | 3 | 31
2016 | 4 | 30
2016 | 5 | 31
2016 | 6 | 30
2016 | 7 | 31
2016 | 8 | 31
2016 | 9 | 30
2016 | 10 | 31
2016 | 11 | 30
2016 | 12 | 31
(12 rows)

Answer 3

This one has better performance since it generates only the last day for each month and consequently does not need aggregation:
select
date_part('year', d) as year,
date_part('month', d) as month,
date_part('day', d) as days_in_month
from
generate_series('2016-01-01'::date, '2016-12-01', '1 month') gs(gsd)
cross join lateral
(select gsd + interval '1 month - 1 day') d(d)
order by 1, 2;
year | month | days_in_month
------+-------+---------------
2016 | 1 | 31
2016 | 2 | 29
2016 | 3 | 31
2016 | 4 | 30
2016 | 5 | 31
2016 | 6 | 30
2016 | 7 | 31
2016 | 8 | 31
2016 | 9 | 30
2016 | 10 | 31
2016 | 11 | 30
2016 | 12 | 31

Answer 4

Another variation, using CTEs for a bit more readability, IMHO (this example generating months and datas for next threee full months following the calendar month of current_date)
WITH series AS (
SELECT generate_series (
date_trunc ('month', date_trunc('day', now()) + interval '1 month'),
date_trunc('day', now() + interval '4 months'), '1d'::interval
) AS day ) SELECT DISTINCT ON (date_part('year', series.day), date_part('month', series.day))
date_part('year', series.day) as year,
date_part('month', series.day) as month,
date_part('day', series.day) as days_in_month
FROM series
ORDER BY 1, 2, 3 desc LIMIT 3;
year | month | days_in_month
------+-------+---------------
2021 | 1 | 31
2021 | 2 | 28
2021 | 3 | 31