21. Sessionize events (30-minute gap)
Asked in screens shaped like: Netflix, Spotify, Airbnb
Group each customer’s events into sessions that break after 30 minutes of inactivity. This is the three-step gap-and-island pattern: look back, flag a break, then accumulate the flag into an id.
Requirements
- Columns: customer_id, occurred_at, session_id
- session_id restarts at 1 for every customer
- A new session starts on the first event or when the gap from the previous event exceeds 30 minutes
- Order by customer_id, then occurred_at
Expected output
customer_id, occurred_at, session_id
Row order is graded, so ORDER BY matters here.
Tables this problem reads. The full warehouse is available in thesandbox.
events
~400 rows
Clickstream. payload is raw JSON — use json_extract on it.
- event_idINTEGER
- customer_idTEXT
- event_nameTEXT— page_view, add_cart, purchase, search
- occurred_atTEXT
- payloadTEXT— {"source":"web|ios","campaign":"c0…c4"}
Pick a warehouse dialect above the editor. The engine is still SQLite; common syntax is rewritten before it runs.
SQLite cheat sheet
- Truncate to day
DATE(ordered_at) - Current date
DATE('now') - Conditional
CASE WHEN … THEN … ELSE … END - String aggregate
GROUP_CONCAT(col, ", ") - JSON field
json_extract(payload, '$.source') - Date difference
JULIANDAY(a) - JULIANDAY(b) - Filter a window
CTE, then WHERE rn = 1 - Median
ROW_NUMBER + COUNT(*) OVER ()
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