SQL Trends in Wait Times | Dataford Interview Questions

Waymo Data Science wants to monitor monthly ride-hail wait times across service zones. Write a PostgreSQL query using data from the past 12 complete months, from September 2025 through August 2026, as of August 29, 2026.

Requirements

Include only completed ride requests in the specified 12-month period and join each request to its service zone.
Calculate monthly average wait time in seconds and the number of requests with recorded wait times for each zone.
Use a window function to compare each zone's average with its previous reported month, and classify the trend as rising, falling, stable, or no_prior_month.
Return results ordered by zone and month. Ignore NULL wait times when calculating averages.

Schema

ride_requests

Column	Type	Description
request_idPK	INT	Unique ride request identifier
zone_id	INT	References service_zones.zone_id
requested_at	TIMESTAMP	Timestamp when the ride was requested
wait_seconds	INT	Observed rider wait time in seconds
status	VARCHAR(20)	Ride request status

service_zones

Column	Type	Description
zone_idPK	INT	Unique service-zone identifier
zone_name	VARCHAR(100)	Waymo service-zone name

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