Count if previous month data exists postgres - sql

Question

i'm stuck with a query to count id where if it exists in previous month than 1
my table look like this
date | id |
2020-02-02| 1 |
2020-03-04| 1 |
2020-03-04| 2 |
2020-04-05| 1 |
2020-04-05| 3 |
2020-05-06| 2 |
2020-05-06| 3 |
2020-06-07| 2 |
2020-06-07| 3 |
i'm stuck with this query
SELECT date_trunc('month',date), id
FROM table
WHERE id IN
(SELECT DISTINCT id FROM table WHERE date
BETWEEN date_trunc('month', current_date) - interval '1 month' AND date_trunc('month', current_date)
the main problem is that i stuck with current_date function. is there any dynamic ways change current_date? thanks
What i expected to be my result is
date | count |
2020-02-01| 0 |
2020-03-01| 1 |
2020-04-01| 1 |
2020-05-01| 1 |
2020-06-01| 2 |

Answer 1

Solution 1 with SELF JOIN
SELECT date_trunc('month', c.date) :: date AS date
, count(DISTINCT c.id) FILTER (WHERE p.date IS NOT NULL)
FROM test AS c
LEFT JOIN test AS p
ON c.id = p.id
AND date_trunc('month', c.date) = date_trunc('month', p.date) + interval '1 month'
GROUP BY date_trunc('month', c.date)
ORDER BY date_trunc('month', c.date)
Result :
date count
2020-02-01 0
2020-03-01 1
2020-04-01 1
2020-05-01 1
2020-06-01 2
Solution 2 with WINDOW FUNCTIONS
SELECT DISTINCT ON (date) date
, count(*) FILTER (WHERE count > 0 AND previous_month) OVER (PARTITION BY date)
FROM
( SELECT DISTINCT ON (id, date_trunc('month', date))
id
, date_trunc('month', date) AS date
, count(*) OVER w AS count
, first_value(date_trunc('month', date)) OVER w = date_trunc('month', date) - interval '1 month' AS previous_month
FROM test
WINDOW w AS (PARTITION BY id ORDER BY date_trunc('month', date) GROUPS BETWEEN 1 PRECEDING AND 1 PRECEDING)
) AS a
Result :
date count
2020-02-01 0
2020-03-01 1
2020-04-01 1
2020-05-01 1
2020-06-01 2
see dbfiddle