Write SQL for a cohort retention analysis (what % of users return in week N after signup).

The classic product-analytics SQL question. The signal is grouping users by signup cohort, deriving each activity's period offset, and counting distinct returners per offset to build the retention triangle.

Updated Sep 2026 · Grounded in real GenAI, LLM, and AI/ML engineering interview loops and written to a senior-engineer editorial bar.