Write a Query for Repeated Rows | Dataford Interview Questions

Zomato’s data quality team wants to identify repeated order-item records that may have been ingested more than once. Treat rows as repeated when all business columns match, excluding the unique order_item_id.

Write a PostgreSQL query to find these repeated rows and enrich the results with the restaurant name.

Requirements

Group records by restaurant, customer, item, quantity, price, and order timestamp.
Return only groups containing at least two rows, including a duplicate count.
Return the matching order_item_id values as a comma-separated list, ordered numerically.
Preserve records whose restaurant ID is not present in the restaurant table.

Schema

order_items

Column	Type	Description
order_item_idPK	INT	Unique order-item record ID
restaurant_id	INT	References restaurants.restaurant_id
customer_id	INT	Customer identifier
item_name	VARCHAR(100)	Ordered menu item
quantity	INT	Quantity ordered
unit_price	DECIMAL(10,2)	Price per item
ordered_at	TIMESTAMP	Order creation timestamp

restaurants

Column	Type	Description
restaurant_idPK	INT	Unique restaurant identifier
restaurant_name	VARCHAR(150)	Restaurant display name

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Problem