Postgres 13

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CREATE TABLE t
    ("date" timestamp, "n_count" int)
;
    
INSERT INTO t 
VALUES
    ('2020-02-19 00:00:00', 4),
    ('2020-07-14 00:00:00', 1),
    ('2020-07-17 00:00:00', 1),
    ('2020-07-30 00:00:00', 2),
    ('2020-08-03 00:00:00', 1),
    ('2020-08-04 00:00:00', 2),
    ('2020-08-25 00:00:00', 2),
    ('2020-09-23 00:00:00', 2),
    ('2020-09-30 00:00:00', 3),
    ('2020-10-01 00:00:00', 11),
    ('2020-10-05 00:00:00', 12),
    ('2020-10-19 00:00:00', 1),
    ('2020-10-20 00:00:00', 1),
    ('2020-10-22 00:00:00', 1),
    ('2020-11-02 00:00:00', 376),
    ('2020-11-04 00:00:00', 72),
    ('2020-11-11 00:00:00', 1)
;

CREATE TABLE



INSERT 0 17


SELECT
    *,
    date_trunc('month', date)
FROM
    t


date
n_count
date_trunc


2020-02-19 00:00:00
4
2020-02-01 00:00:00

2020-07-14 00:00:00
1
2020-07-01 00:00:00

2020-07-17 00:00:00
1
2020-07-01 00:00:00

2020-07-30 00:00:00
2
2020-07-01 00:00:00

2020-08-03 00:00:00
1
2020-08-01 00:00:00

2020-08-04 00:00:00
2
2020-08-01 00:00:00

2020-08-25 00:00:00
2
2020-08-01 00:00:00

2020-09-23 00:00:00
2
2020-09-01 00:00:00

2020-09-30 00:00:00
3
2020-09-01 00:00:00

2020-10-01 00:00:00
11
2020-10-01 00:00:00

2020-10-05 00:00:00
12
2020-10-01 00:00:00

2020-10-19 00:00:00
1
2020-10-01 00:00:00

2020-10-20 00:00:00
1
2020-10-01 00:00:00

2020-10-22 00:00:00
1
2020-10-01 00:00:00

2020-11-02 00:00:00
376
2020-11-01 00:00:00

2020-11-04 00:00:00
72
2020-11-01 00:00:00

2020-11-11 00:00:00
1
2020-11-01 00:00:00



SELECT 17


WITH cte AS (
    SELECT
       *,
        date_trunc('month', date)::date AS time
    FROM
        t
)
SELECT
    time,
    n_count
FROM cte

UNION

SELECT 
    generate_series(
        (SELECT MIN(time) FROM cte),
        (SELECT MAX(time) FROM cte),
        interval '1 month'
    )::date,
    0


time
n_count


2020-10-01
0

2020-11-01
1

2020-11-01
72

2020-06-01
0

2020-11-01
0

2020-07-01
1

2020-02-01
0

2020-07-01
2

2020-08-01
2

2020-10-01
11

2020-09-01
3

2020-09-01
2

2020-02-01
4

2020-08-01
0

2020-10-01
12

2020-07-01
0

2020-08-01
1

2020-10-01
1

2020-09-01
0

2020-03-01
0

2020-04-01
0

2020-11-01
376

2020-05-01
0



SELECT 23


WITH cte AS (
    SELECT
        *,
        date_trunc('month', date)::date AS time
    FROM
        t
)
SELECT 
    time,
    SUM(n_count) as month_count      
FROM (
    SELECT
        time,
        n_count
    FROM cte

    UNION

    SELECT 
        generate_series(
            (SELECT MIN(time) FROM cte),
            (SELECT MAX(time) FROM cte),
            interval '1 month'
        )::date,
        0
) s
GROUP BY time
ORDER BY time


time
month_count


2020-02-01
4

2020-03-01
0

2020-04-01
0

2020-05-01
0

2020-06-01
0

2020-07-01
3

2020-08-01
3

2020-09-01
5

2020-10-01
24

2020-11-01
449



SELECT 10

date	n_count	date_trunc
2020-02-19 00:00:00	4	2020-02-01 00:00:00
2020-07-14 00:00:00	1	2020-07-01 00:00:00
2020-07-17 00:00:00	1	2020-07-01 00:00:00
2020-07-30 00:00:00	2	2020-07-01 00:00:00
2020-08-03 00:00:00	1	2020-08-01 00:00:00
2020-08-04 00:00:00	2	2020-08-01 00:00:00
2020-08-25 00:00:00	2	2020-08-01 00:00:00
2020-09-23 00:00:00	2	2020-09-01 00:00:00
2020-09-30 00:00:00	3	2020-09-01 00:00:00
2020-10-01 00:00:00	11	2020-10-01 00:00:00
2020-10-05 00:00:00	12	2020-10-01 00:00:00
2020-10-19 00:00:00	1	2020-10-01 00:00:00
2020-10-20 00:00:00	1	2020-10-01 00:00:00
2020-10-22 00:00:00	1	2020-10-01 00:00:00
2020-11-02 00:00:00	376	2020-11-01 00:00:00
2020-11-04 00:00:00	72	2020-11-01 00:00:00
2020-11-11 00:00:00	1	2020-11-01 00:00:00

time	n_count
2020-10-01	0
2020-11-01	1
2020-11-01	72
2020-06-01	0
2020-11-01	0
2020-07-01	1
2020-02-01	0
2020-07-01	2
2020-08-01	2
2020-10-01	11
2020-09-01	3
2020-09-01	2
2020-02-01	4
2020-08-01	0
2020-10-01	12
2020-07-01	0
2020-08-01	1
2020-10-01	1
2020-09-01	0
2020-03-01	0
2020-04-01	0
2020-11-01	376
2020-05-01	0