SQL for Top Usage Customers | Dataford Interview Questions

Write a SQL query to identify the top 10% of customers based on specific usage metrics.

Use usage records from calendar year 2025. Rank every customer by completed usage units, then active usage days and completed usage events as tie-breakers. Include customers with no qualifying usage, and return only the highest decile.

Output

One row per customer in the top 10%.
Columns: customer_id, customer_name, total_usage_units, active_usage_days, completed_usage_events, and usage_decile.
Sort by total usage units descending, then customer ID ascending.

Schema

customers

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

usage_events

Column	Type	Description
event_idPK	INT	Unique usage event identifier
customer_id	INT	Customer associated with the usage event
event_date	DATE	Date on which usage occurred
usage_units	INT	Units recorded for the usage event
status	VARCHAR(20)	Event processing status

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Problem