SQL - Identify if a user is present every month - sql

Question

I am performing some data analysis on users who have made transactions over the course of three months.
What I would like to do is identify customers who made specific transaction types (Credit) in every single month present in the data table over those two years. As you can see in the data table below, User A has performed a Credit transaction in months 1,2,3 and I would like a flag saying "Frequent" applied to the customer.
User B, however, has not performed a credit transaction every month (month 2 was Debit), and so I would like them to have a different flag name (e.g. "Infrequent").
How can I use SQL to identify if a user has made a specific transaction type each month?
| Date | User | Amount | Transaction Type | **Flag ** |
| 2022-01-15 | A | $15.00 | Credit | **Flag ** |
...
| 2022-02-15 | A | $15.00 | Credit | **Flag ** |
...
| 2022-03-15 | A | $15.00 | Credit | **Flag ** |
...
...
| 2022-01-15 | B | $15.00 | Credit | **Flag ** |
...
| 2022-02-15 | B | $15.00 | Debit | **Flag ** |
...
| 2022-03-15 | B | $15.00 | Credit | **Flag ** |
I have tried the following - hoping there is a better or more simple way.
SELECT
Date, User, Amount, Transaction_Type,
CASE WHEN Count(present) = 3 THEN 'Frequent' ELSE 'Infrequent'
FROM Transactions
LEFT JOIN (
SELECT
User,Month(Date),Count(Transaction_Type) as present
FROM
Transactions
WHERE
Transaction_Type = 'Credit'
GROUP BY
User,Month(Date)
Having
Count(Transaction_Type) > 0
) subquery
ON subquery.User = Transaction.User
GROUP BY
Date,User,Amount,Transaction_Type

Answer 1

That is the way I would approach it. Assuming you are using T-SQL I would make the following changes. Instead of having the LEFT JOIN be to a sub-query, I would make the sub-query a CTE and then joint to that. I find it easier to grok when the main query is not full of sub-queries and you can test the CTE on its own more easily, plus if performance becomes an issue is relatively trivial to convert the CTE to a temp table. without affecting the main query too much.
You have a couple of problems I think. the first is that your subquery is going to return you the count of the credits in each month. If I make 3 credits in January this is going to flag me as frequent because the total is more than 3. You probably want to do a
COUNT(DISTINCT Transaction_type) AS hasCredit
to identify if there is AT LEAST ONE credit transaction, then have another aggregation that
SUM(hasCredit)
to get the number of months in which a credit appears.
using nested sub-queries means your LEFT JOIN would now be two sub-queries deep and dissapearing off the right hand side of your screen. Writing them as CTEs keeps the main logic clean and script narrow.
I think this does what you need, but can't test it because I don't have any sample data.
WITH CTE_HasCredit AS
(
SELECT
User
,Month(Date) AS [TransactionMonth]
,Count(DISTINCT Transaction_Type) AS [hasCredit]
FROM
Transactions
WHERE
Transaction_Type = 'Credit'
GROUP BY
User
,Month(Date)
Having
Count(Transaction_Type) > 0
)
,
CTE_isFrequent AS
(
SELECT
User
,SUM(hasCredit) AS [TotalCredits]
FROM
CTE_HasCredit
GROUP BY
User
)
SELECT
TXN.Date
, TXN.User
, TXN.Amount
, TXN.Transaction_Type
,CASE
WHEN FRQ.TotalCredits >= 3 THEN 'Frequent'
ELSE 'Infrequent'
END AS [customerType]
FROM
Transactions AS TXN
LEFT JOIN
CTE_isFrequent AS FRQ ON FRQ.User = TXN.User
GROUP BY
TXN.Date
,TXN.User
,TXN.Amount
,TXN.Transaction_Type
I don't think you need the GROUP BY on the main query either; it would de-dupe transactions for the same day for the same amount.
You might also want to look at the syntax for COUNT() OVER(). These would allow you to do the calculations in the main query and would look something like.
,CASE
WHEN COUNT(DISTINCT TXN.Transaction_Type) OVER(PARTITION BY User, MONTH(TXN.Date),TXN.Transaction_Type) >=3 THEN 'Frequent'
ELSE 'Infrequent'
END AS [customerType2]
This second way would give you customer type for both the Debits and Credits. I am not aware of a way to filter the COUNT() OVER() to just Credits, for that you would need to use the CTE method.