Tables Used

transaction

Column	Type
transactionid	bigint
orderid	bigint
customerid	bigint
supplierid	bigint
transactiontype	text
amount	double precision
currencyid	bigint
transactiondate	text
paymentmethod	text
description	text
status	text
createdat	text
updatedat	text

orders

Column	Type
orderid	bigint
customerid	bigint
orderdate	timestamp without time zone
totalamount	double precision
status	text

Expected Output

Column
customerid
transactiondate
amount
previouspayment

Payment Patterns Revealed :Compare Each Customer’s Current and Previous Payment Amounts Over Time Intermediate

Q: Do I need advanced SQL to attempt this?

This problem is labeled as Intermediate. It assumes you’ve written a few window function queries before, but you don’t need to be an expert. Use the hints and explanations if you get stuck — they’re there to help you think through the logic.

Track each customer's past and present payments using lag().

Xorthax is curious about payment patterns among his customers. For each payment made, show the current amount and the amount of the previous payment by the same customer.

Write Your Query

Answer:

          select o.customerid,
       t.transactiondate,
       t.amount,
       lag(t.amount) over (partition by o.customerid
                           order by t.transactiondate) as previouspayment
from transaction t
join orders o on t.orderid = o.orderid;

This mirrors financial tracking where analysts monitor how individual customer payments change across consecutive transactions.

You’ll practice ordering and comparing related records within customer payment histories to identify sequential value changes.

This problem is labeled as Intermediate. It assumes you’ve written a few window function queries before, but you don’t need to be an expert. Use the hints and explanations if you get stuck — they’re there to help you think through the logic.

Yes. Every problem comes with optional hints you can reveal one at a time, plus a fully worked step-by-step solution. You decide how much help you want while practicing.

All problems on PracticeWindowFunctions.com are completely free and can be solved without creating an account. Right now there are over 80 practice problems, with new ones added regularly.

SQL for Data Analysis (O’Reilly)

SQL techniques tailored for analytics and reporting workflows.

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Tables Used

Expected Output

Payment Patterns Revealed :Compare Each Customer’s Current and Previous Payment Amounts Over Time Intermediate

Track each customer's past and present payments using lag().

What real-world scenario does this represent?

What SQL concepts will I practice here?

Do I need advanced SQL to attempt this?

Will I get hints and step-by-step explanations if I get stuck?

Are the problems free to access, and how many are available?

SQL for Data Analysis (O’Reilly)