Given rows with start and end timestamps, merge all overlapping intervals per user in SQL.
Overlapping subscription or session windows have to collapse into clean periods, and a naive self-join runs quadratic. The signal is the gaps-and-islands trick: a running max end-time that marks where each new island begins.
Updated Sep 2026 · Grounded in real GenAI, LLM, and AI/ML engineering interview loops and written to a senior-engineer editorial bar.
Overlapping subscription or session windows have to collapse into clean periods, and a naive self-join runs quadratic. The signal is the gaps-and-islands trick: a running max end-time that marks where each new island begins.
Lead with where the obvious approach breaks, because that is the judgment they are screening for — most candidates jump straight to the happy path and lose the room.
Then walk the failure back through the pipeline in order, naming the one metric the customer's exec sponsor actually cares about before you propose the fix.
