Who Stayed
A hard SQL mock interview question on DataDriven. Practice with AI-powered feedback, real code execution, and a hire/no-hire decision.
- Domain
- SQL
- Difficulty
- hard
- Seniority
- L5
Interview Prompt
We run periodic health checks across a fleet of services, and every check records the region it ran in. For December 2025 and January 2026, we want to know how much of each service's regional footprint holds up: of the regions a service was checked in during the month, what share get checked again for that same service in some later month. Return the service, the month, and that share as a percentage, treating a month whose regions never reappear as zero.
Summary
A footprint either holds or it slips away. Find how much stays.
How This Interview Works
- Read the vague prompt (just like a real interview)
- Ask clarifying questions to the AI interviewer
- Write your sql solution with real code execution
- Get instant feedback and a hire/no-hire decision