Top Merchants by Transaction Volume | Dataford Interview Questions

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

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