Session Detection — Solution & ExplanationProblems / Session Detection / EditorialApproachSession detection is a classic window function problem with a three-step pattern:

LAG: Get each event's previous event time within the same user.

Flag: If the gap exceeds 30 minutes (or there is no previous event), mark it as a new session (1), otherwise 0.

Running SUM: A cumulative sum of the flag produces a monotonically increasing session ID that increments at each session boundary.

This is widely used in clickstream analytics, app analytics, and user behavior modeling.Hints

Use LAG to get each event's previous timestamp, then compute the time gap.

Create a binary flag (1 = new session, 0 = same session) based on the 30-minute threshold.

A running SUM of the new_session flag gives a sequential session_id.

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