Count Same IDs in two different Date Periods - postgresql

Question

Let's say I want the ids that are counted as a result of my query, and checked if the same ids appear in a different month.
Here I join 2 tables via distinct id's and count the returning rows to know how many of the matching id's I have. Here that is for the month June.
I'd like:
eg. in June 100 distinct ids
eg. in July 90 of the same ids left
Please help!
I am stuck as my Sql is not very advanced,...
with total as (
select distinct(transactions.u_id), count(*)
from transactions
join contacts using (u_id)
join table using (contact_id)
where transactions.when_created between '2020-06-01' AND '2020-06-30'
group by transactions.u_id
HAVING COUNT(*) > 1
)
SELECT
COUNT(*)
FROM
total

Answer 1

Let's say that you are interested about the query of the like of
select transactions.u_id, count(*) as total
from transactions
join contacts using (u_id)
join table using (contact_ud)
where transactions.when_created between '2020-06-01' and '2020-06-30'
group by transactions.u_id;
You are also interested in
select transactions.u_id, count(*) as total
from transactions
join contacts using (u_id)
join table using (contact_ud)
where transactions.when_created between '2020-08-01' and '2020-08-30'
group by transactions.u_id;
And you want to get:
the ids which can be found by both
the minimum total
Then, you can do something of the like of
select t1.u_id,
case
when t1.total > t2.total then t2.total
else t1.total
end as total
from (
select transactions.u_id, count(*) as total
from transactions
join contacts using (u_id)
join table using (contact_ud)
where transactions.when_created between '2020-06-01' and '2020-06-30'
group by transactions.u_id) t1
join (
select transactions.u_id, count(*) as total
from transactions
join contacts using (u_id)
join table using (contact_ud)
where transactions.when_created between '2020-06-01' and '2020-06-30'
group by transactions.u_id) t2
on t1.u_id = t2.u_id