Who Stayed

A hard SQL interview practice problem on DataDriven. Write and execute real sql code with instant grading.

Domain

SQL

Difficulty

hard

Seniority

L5

Problem

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.

Practice This Problem

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