Top Merchants by Transaction Volume | Dataford Interview Questions

Your question is Top Merchants by Transaction Volume. Start with the requirements and the two tables on the right.

Run and submit as often as you like. When you're ready, talk me through your approach or go straight to the code.

Problem

PayU needs a monthly view of its highest-volume merchants on the PayU payment platform. Write a PostgreSQL query that identifies the top three merchants by the number of successful transactions during January 2025.

Requirements

Count only transactions with status = 'SUCCESS' during January 2025, using an inclusive start date and exclusive end date.
Include merchants with no qualifying transactions, assigning them a volume of zero.
Rank merchants by successful transaction volume in descending order, breaking ties with the lower merchant_id.
Return only the top three ranked merchants, including their rank and transaction count.

Schema

merchants

Column	Type	Description
merchant_idPK	INT	Unique merchant identifier
merchant_name	VARCHAR(100)	Merchant display name
business_category	VARCHAR(50)	Merchant business category

transactions

Column	Type	Description
transaction_idPK	BIGINT	Unique transaction identifier
merchant_id	INT	Merchant associated with the transaction
transaction_timestamp	TIMESTAMP	Timestamp when the transaction was created
status	VARCHAR(20)	Transaction processing status
payment_method	VARCHAR(30)	Payment method used