Request Failure Rate by Data Center | Dataford Interview Questions

Given user_network_requests (userid, timestamp, data_center, success) and user_country (userid, country), calculate the request failure rate for each data center.

Asked in the Phone Screen stage. SQL question asked during the technical phone screen.

Task

Write a SQL query that returns the failure rate for each data_center in Meta's request logs.

Requirements

Join the request log to user_country on userid.
Compute failure rate as failed requests divided by total requests per data_center.
Return one row per data_center, ordered by highest failure rate first.
Use PostgreSQL-compatible SQL.

Schema

user_network_requests

Column	Type	Description
userid	INT	Meta user identifier
timestamp	TIMESTAMP	Request timestamp
data_center	VARCHAR(50)	Serving data center name
success	BOOLEAN	Whether the request succeeded

user_country

Column	Type	Description
userid	INT	Meta user identifier
country	VARCHAR(50)	User country

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Problem