Calculating Session Duration
In this lesson, you will learn how to measure the total time a user spends on your platform by aggregating individual event timestamps into coherent session windows.
The Logic of Sessions
A session is typically defined as a series of interactions by a single user where no two consecutive actions are separated by more than a specific duration (e.g., 30 minutes of inactivity). To calculate the total duration, we must first identify the start and end of each session and then compute the difference between those timestamps.
If we have a table events containing user_id and timestamp, we can determine the duration by finding the minimum and maximum timestamps for each session group:
SELECT
user_id,
session_id,
MAX(timestamp) - MIN(timestamp) AS session_duration
FROM user_events
GROUP BY user_id, session_id;
Challenges in Grouping
The complexity arises when the session_id isn't explicitly provided in your data. In such cases, you must generate one. You can use a window function to look at the previous event's timestamp:
- Calculate the Gap: Use
LAG()to see the time difference between the current row and the previous row for that specific user. - Identify Boundaries: Create a "flag" (1 or 0) that turns on whenever the gap exceeds your inactivity threshold.
- Generate IDs: Use a
SUM()window function to create a running total of those flags. This acts as a grouping key, effectively assigning a unique ID to every continuous block of activity.
Once you have this "Session ID," calculating the duration is simply a matter of performing a grouped aggregation (MAX - MIN) on the timestamps within those sessions. This allows you to understand user engagement depth beyond simple page-view counts, revealing how long users actually linger during their visits.
Key Takeaway: Calculating session duration requires defining a gap threshold to segment raw event logs into distinct user sessions, followed by a grouped subtraction of their minimum and maximum timestamps.