Aggregations in SQLite

I am looking to construct a SQL query which is able to sum up installment values by month. Ordinarily this wouldn’t be too hard as you would simply sum the installment values and group by month. However, the problem is not that easy and in the rest of the post, I’ll illustrate why and solicit any help people are able to offer.

First it’s important to note the installments column. Where installments is 1, that means the total value is paid at the time of the purchase. Where installments is greater than 1, this means that the total value is paid in the current month as well as the following months. For example, if we see transaction_id 9 and 10, this is a $100 transaction that has 2 installments which means that $50 will be paid in February and $50 will be paid in March.

Consider that we want to see the monthly bills for credit_card_id = 11111111. If we look to the installments column, we can see that the correct output should be as follows:

January: 19.99 + 75.3
February: 1337 + 75.3
March: 75.3

Again, just to be clear, the 75.3 in March occurs because we had a 3 installment transaction in January meaning the customer would be charged 75.3 in January, February, and March. The issue is that I don’t know how to create the category for March from the given data.

First, I re-created the table in SQL and was easily able to get all the transactions for the card by month with the following SQLite query

select strftime('%m', transaction_date) as Month, total_value, installment_value, installments 
from transactions 
WHERE credit_card_id = '11111111';

which outputs a table that looks like this

However, it wasn’t obvious how to split the 3 installment periods into 01, 02, and 03, so I created a new table with a column txn which is meant to give an id to the unique transactions that can be thought of as 1 group.

CREATE TABLE transactions (
    transaction_id int primary key,
    credit_card_id int,
    transaction_date timestamp,
    merchant_name varchar(256),
    total_value decimal(19,4),
    installment_value decimal(19,4),
    installments int,
    txn int
);

insert into transactions values(1,11111111,'2018-01-10T00:00:00','Colorful Soaps', 19.99, 19.99, 1, 1);
insert into transactions values(2,22222222,'2018-01-11T00:01:00','Cantina da Mamma',43.5,43.5,1,2);
insert into transactions values(3,33333333,'2018-01-12T01:02:00','Boulevard Hotel',129,129,1,3);
insert into transactions values(4,11111111,'2018-01-15T11:11:11','Micas Bar',225.9,75.3,3,4);
insert into transactions values(5,11111111,'2018-01-15T11:11:11','Micas Bar',225.9,75.3,3,4);
insert into transactions values(6,11111111,'2018-01-15T11:11:11','Micas Bar',225.9,75.3,3,4);
insert into transactions values(7,22222222,'2018-01-18T22:10:01','IPear Store',9999.99,9999.99,1,5);
insert into transactions values(8,11111111,'2018-02-20T21:08:32','Forrest Paintball',1337,1337,1,6);
insert into transactions values(9,44444444,'2018-02-22T00:05:30','Unicorn Costumes',100,50,2,7);
insert into transactions values(10,44444444,'2018-02-22T00:05:30','Unicorn Costumes',100,50,2,7);

My questions are

Is it possible to get an output of the format I identified above in SQLite and if so, how?
Do I have to have the txn column in order to get this information?

Thank you for your help.

Answer

Assuming you run SQLite 3.25+ version, consider using CTE and window function that creates a running count by same credit_card_id and transaction_date and uses this value to add needed months to transaction date. From there, aggregate according to new calculated date, install_date.

WITH cte AS
   (SELECT *,
       DATE(transaction_date, 
            '+' || (ROW_NUMBER() 
                      OVER(PARTITION BY transaction_date, credit_card_id 
                           ORDER BY transaction_date) - 1)
                || ' month'
            ) AS install_date       
    FROM transactions)

SELECT credit_card_id, 
       STRFTIME('%Y', install_date) AS install_year, 
       STRFTIME('%m', install_date) AS install_month, 
       SUM(installment_value) AS sum_installment_value
FROM cte 
GROUP BY credit_card_id, 
         STRFTIME('%Y', install_date), 
         STRFTIME('%m', install_date)
ORDER BY credit_card_id, 
         STRFTIME('%Y', install_date), 
         STRFTIME('%m', install_date);

Rextester Demo using PostgreSQL, since AFAIK no online fiddle (SQLFiddle, SQLiteonline, DBFiddle, etc.) supports SQLite with window functions

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Answer