BB_1

/*create table oper (date date, cnt int)
insert into oper values
('2019-06-02',     1985),
('2019-06-03',     1577),
('2019-06-04',     1597),
('2019-06-05',     1468),
('2019-07-06',     82),
('2019-07-08',     1689),
('2019-07-09',     1556),
('2019-07-10',     1480),
('2019-07-11',     1405),
('2019-07-12',     1502)*/
SELECT date, SUM(cnt) OVER(PARTITION BY DATE_TRUNC('month', date) ORDER BY date)
FROM oper