Top 10 Customers by Sales | Dataford Interview Questions

HCL needs a sales ranking for customers using data from its commerce platform. Write a PostgreSQL query that identifies the top 10 customers by sales from completed orders.

Requirements

Join customers, orders, and order_items to calculate sales as quantity * unit_price.
Include only orders with status = 'completed'; treat a NULL unit price as zero.
Aggregate sales per customer, return the top 10, and break equal-sales ties by ascending customer_id.

Schema

customers

Column	Type	Description
customer_idPK	INT	Unique customer identifier
customer_name	VARCHAR(100)	Customer display name

orders

Column	Type	Description
order_idPK	INT	Unique order identifier
customer_id	INT	Customer associated with the order
status	VARCHAR(20)	Current order status

order_items

Column	Type	Description
order_item_idPK	INT	Unique order-line identifier
order_id	INT	Order containing the item
quantity	INT	Number of units purchased
unit_price	NUMERIC(10,2)	Price per unit at purchase time

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