test=*# select * from bla;
monat | wert
------------+------
2021-01-01 | 123
2021-02-01 | 456
2021-03-01 | 678
(3 rows)
test=*# with dates as (select datum::date from generate_series((select min(monat) from bla),(select (max(monat) + '1month'::interval)::date from bla) - '1day'::interval, '1day') datum), tage as (select date_trunc('month', datum)::date monat, count(1) as anzahl from dates group by 1) select dates.datum, bla.wert, bla.wert / tage.anzahl as tageswert from dates left join bla on (date_trunc('month',dates.datum)=date_trunc('month',bla.monat)) left join tage on (tage.monat=date_trunc('month',bla.monat));
datum | wert | tageswert
------------+------+---------------------
2021-01-01 | 123 | 3.9677419354838710
2021-01-02 | 123 | 3.9677419354838710
2021-01-03 | 123 | 3.9677419354838710
2021-01-04 | 123 | 3.9677419354838710
2021-01-05 | 123 | 3.9677419354838710
2021-01-06 | 123 | 3.9677419354838710
2021-01-07 | 123 | 3.9677419354838710
2021-01-08 | 123 | 3.9677419354838710
2021-01-09 | 123 | 3.9677419354838710
2021-01-10 | 123 | 3.9677419354838710
2021-01-11 | 123 | 3.9677419354838710
2021-01-12 | 123 | 3.9677419354838710
2021-01-13 | 123 | 3.9677419354838710
2021-01-14 | 123 | 3.9677419354838710
2021-01-15 | 123 | 3.9677419354838710
2021-01-16 | 123 | 3.9677419354838710
2021-01-17 | 123 | 3.9677419354838710
2021-01-18 | 123 | 3.9677419354838710
2021-01-19 | 123 | 3.9677419354838710
2021-01-20 | 123 | 3.9677419354838710
2021-01-21 | 123 | 3.9677419354838710
2021-01-22 | 123 | 3.9677419354838710
2021-01-23 | 123 | 3.9677419354838710
2021-01-24 | 123 | 3.9677419354838710
2021-01-25 | 123 | 3.9677419354838710
2021-01-26 | 123 | 3.9677419354838710
2021-01-27 | 123 | 3.9677419354838710
2021-01-28 | 123 | 3.9677419354838710
2021-01-29 | 123 | 3.9677419354838710
2021-01-30 | 123 | 3.9677419354838710
2021-01-31 | 123 | 3.9677419354838710
2021-02-01 | 456 | 16.2857142857142857
2021-02-02 | 456 | 16.2857142857142857
2021-02-03 | 456 | 16.2857142857142857
2021-02-04 | 456 | 16.2857142857142857
2021-02-05 | 456 | 16.2857142857142857
2021-02-06 | 456 | 16.2857142857142857
2021-02-07 | 456 | 16.2857142857142857
2021-02-08 | 456 | 16.2857142857142857
2021-02-09 | 456 | 16.2857142857142857
2021-02-10 | 456 | 16.2857142857142857
2021-02-11 | 456 | 16.2857142857142857
2021-02-12 | 456 | 16.2857142857142857
2021-02-13 | 456 | 16.2857142857142857
2021-02-14 | 456 | 16.2857142857142857
2021-02-15 | 456 | 16.2857142857142857
2021-02-16 | 456 | 16.2857142857142857
2021-02-17 | 456 | 16.2857142857142857
2021-02-18 | 456 | 16.2857142857142857
2021-02-19 | 456 | 16.2857142857142857
2021-02-20 | 456 | 16.2857142857142857
2021-02-21 | 456 | 16.2857142857142857
2021-02-22 | 456 | 16.2857142857142857
2021-02-23 | 456 | 16.2857142857142857
2021-02-24 | 456 | 16.2857142857142857
2021-02-25 | 456 | 16.2857142857142857
2021-02-26 | 456 | 16.2857142857142857
2021-02-27 | 456 | 16.2857142857142857
2021-02-28 | 456 | 16.2857142857142857
2021-03-01 | 678 | 21.8709677419354839
2021-03-02 | 678 | 21.8709677419354839
2021-03-03 | 678 | 21.8709677419354839
2021-03-04 | 678 | 21.8709677419354839
2021-03-05 | 678 | 21.8709677419354839
2021-03-06 | 678 | 21.8709677419354839
2021-03-07 | 678 | 21.8709677419354839
2021-03-08 | 678 | 21.8709677419354839
2021-03-09 | 678 | 21.8709677419354839
2021-03-10 | 678 | 21.8709677419354839
2021-03-11 | 678 | 21.8709677419354839
2021-03-12 | 678 | 21.8709677419354839
2021-03-13 | 678 | 21.8709677419354839
2021-03-14 | 678 | 21.8709677419354839
2021-03-15 | 678 | 21.8709677419354839
2021-03-16 | 678 | 21.8709677419354839
2021-03-17 | 678 | 21.8709677419354839
2021-03-18 | 678 | 21.8709677419354839
2021-03-19 | 678 | 21.8709677419354839
2021-03-20 | 678 | 21.8709677419354839
2021-03-21 | 678 | 21.8709677419354839
2021-03-22 | 678 | 21.8709677419354839
2021-03-23 | 678 | 21.8709677419354839
2021-03-24 | 678 | 21.8709677419354839
2021-03-25 | 678 | 21.8709677419354839
2021-03-26 | 678 | 21.8709677419354839
2021-03-27 | 678 | 21.8709677419354839
2021-03-28 | 678 | 21.8709677419354839
2021-03-29 | 678 | 21.8709677419354839
2021-03-30 | 678 | 21.8709677419354839
2021-03-31 | 678 | 21.8709677419354839
(90 rows)