Get the records from previous year from same account ids present at both time periods

Question

I am trying to get count of transactions for years 2019 and 2021 and compare them with corresponding months for the same customers who have been in 2019 and 2021.example - compare users from 2019 jan who are also there in 2021 Jan and count their transactions and then compare them. I have tried to do a self join but I am getting same number of transactions for both the transactions, any help on tweaking or telling where I have written the query wrongly will help me a lot.

Select datepart(r2021.transdate, year), datepart(r2021transdate,month)
,Count (r2021.transaction_id)  as transidcount2021
Count (r2019.transaction_id) as transidcount2019

From rewards r2021
Join rewards r2019
On datepart(r2021.transdate, year)=datepart(r2019.transdate, year)+2
And 
datepart(r2021transdate,month) =datepart(r2019transdate,month)
And
R2021.accountid = r2019.accountid
Where 
datepart(r2021.transdate, year) =2021
Group by 1,2

Now if I run this I get same values for transcount2019 and transcount2021 for all months in 2021.

Answer 1

compare users from 2019 jan who are also there in 2021 Jan and count their transactions and then compare them

This suggests that you want a table with columns such as:

• month of the year
• users in 2019
• transactions in 2019
• users in 2021
• transactions in 2021
• users in both years
• transactions in both years

That suggests two levels of aggregation:

select month,
       count(*) filter (where cnt_2019 > 0) as customers_2019,
       sum(cnt_2019) as transactions_2019,
       count(*) filter (where cnt_2021 > 0) as customers_2021,
       sum(cnt_2021) as transactions_2021,
       count(*) filter (where cnt_2019 > 0 and cnt_2021 > 0) as customers_both,
       sum(cnt_2019 + cnt_2021) filter (where cnt_2019 > 0 and cnt_2021 > 0) as transactions_both
from (select accountid, extract(month from trandate) as month,
             count(*) filter (where extract(year from trandate) = 2019) as cnt_2019,
             count(*) filter (where extract(year from trandate) = 2021) as cnt_2021
      from rewards r
      group by accountid, month
     ) am
group by month;