Optimize Slow SQL Queries | Dataford Interview Questions

How would you approach optimizing a slow-running SQL query?

Using the supplied PostgreSQL tables, demonstrate your approach with a query that returns completed order totals for customers during January 2025. Keep the response focused on query structure, filtering, join behavior, and performance considerations.

Output

One row per qualifying customer.
Columns: customer_id, customer_name, total_completed_amount.
Include only customers whose January completed order total is greater than zero.
Sort by total amount descending, then customer ID ascending.

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)	Order processing status
order_date	DATE	Date the order was placed
amount	DECIMAL(10,2)	Order amount

Your question is Optimize Slow SQL Queries. Start with the requirements and the two tables in the Question tab.

Run and submit as often as you like. When you're ready, talk me through your approach or go straight to the code.

Problem