Group By and Having for Duplicates | Dataford Interview Questions

Systems Planning and Analysis needs to audit records in its transaction ledger for duplicate business events received from multiple ingestion sources. Write a PostgreSQL query that identifies duplicate transaction groups without relying on the unique transaction_id.

Requirements

Define duplicates as rows sharing account_number, transaction_date, merchant_code, amount, and currency.
Use GROUP BY and HAVING COUNT(*) > 1 to find duplicate groups.
Return the duplicate count, sorted transaction IDs, and distinct source names. Preserve duplicate records whose source_id has no matching source record.
Treat NULL merchant_code values as equal for duplicate detection.

Schema

transactions

Column	Type	Description
transaction_idPK	INT	Unique transaction record ID
account_number	VARCHAR(20)	SPA ledger account reference
transaction_date	DATE	Business transaction date
merchant_code	VARCHAR(20)	Merchant identifier
amount	NUMERIC(12,2)	Transaction amount
currency	VARCHAR(3)	ISO currency code
source_id	INT	Ingestion source reference

transaction_sources

Column	Type	Description
source_idPK	INT	Source identifier
source_name	VARCHAR(80)	Ingestion source name

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Problem