SQL Window Session Boundaries | Dataford Interview Questions

Credit Karma wants to analyze how customers navigate its product experience. Write a PostgreSQL query that orders each customer's events chronologically, identifies session starts, and calculates the elapsed time since the previous event.

Requirements

Use a window function to retrieve the previous event for each customer.
Mark an event as a new session when it is the customer's first event or occurs more than 30 minutes after the previous event.
Calculate the time since the previous event in minutes, preserving NULL for each customer's first event.
Include the customer's name when available, retain events without a matching customer record, and order the final output by customer and event time.

Schema

customers

Column	Type	Description
customer_idPK	INTEGER	Unique customer identifier
customer_name	VARCHAR(100)	Customer's display name
email	VARCHAR(255)	Customer email address

customer_events

Column	Type	Description
event_idPK	INTEGER	Unique event identifier
customer_id	INTEGER	Customer associated with the event
event_ts	TIMESTAMP	Time when the event occurred
event_type	VARCHAR(40)	Type of customer activity
channel	VARCHAR(30)	Access channel used for the event

Your question is SQL Window Session Boundaries. Start with the requirements and the two tables in the Question tab.

Run and submit as often as you like. When you're ready, talk me through your approach or go straight to the code.

Problem