SQL Performance for Joins | Dataford Interview Questions

How would you optimize a slow-running SQL query involving multiple joins and aggregations?

Use the PostgreSQL tables provided to return completed orders from the first quarter of 2025. The query should calculate customer-level order counts and revenue while avoiding unnecessary row multiplication.

Output

One row per qualifying customer with customer_id, customer_name, region, total_orders, and total_revenue
Include only customers with positive revenue from completed orders in the specified period
Order by total_revenue descending, then customer_id ascending

Schema

customers

Column	Type	Description
customer_idPK	INT	Unique customer identifier
customer_name	VARCHAR(100)	Customer display name
region	VARCHAR(50)	Customer geographic region

orders

Column	Type	Description
order_idPK	INT	Unique order identifier
customer_id	INT	Customer associated with the order
order_date	DATE	Date the order was placed
status	VARCHAR(20)	Order processing 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 on the order line
unit_price	NUMERIC(10,2)	Price per unit

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Problem