Ad-Free Sessions

Ad-Free Sessions

What is this problem about?

The Ad-Free Sessions interview question involves comparing two timelines. You have a table of user Playback sessions (with start and end times) and a table of Ads (with a timestamp of when they were shown). You need to find all session IDs where no advertisement was shown during the session.

Why is this asked in interviews?

Amazon uses this Ad-Free Sessions coding problem to test your ability to perform non-equi joins. Most joins are based on equality (A.id = B.id), but this problem requires joining or filtering based on an inequality or range (timestamp BETWEEN start_time AND end_time).

Algorithmic pattern used

This utilizes the Anti-Join interview pattern. You want to find all sessions that do not have a corresponding entry in the Ads table that falls within their time range. This is usually implemented using a LEFT JOIN where the right side is NULL, or a NOT EXISTS clause.

Example explanation

• Session 1: 10:00 to 10:30
• Ad A: 10:15
• Session 2: 11:00 to 11:30
• Ad B: 12:00

For Session 1, Ad A falls between the start and end. For Session 2, no ads fall in the range. Session 2 is "Ad-Free."

Common mistakes candidates make

• Inefficient Joins: Using a Cross Join and then filtering, which is very slow on large datasets.
• Range Overlap Logic: Overcomplicating the logic. A simple timestamp >= start_time AND timestamp <= end_time inside a NOT EXISTS subquery is usually sufficient.
• Handling Nulls: If using NOT IN, being unaware that if the subquery returns any NULL values, the entire expression may return false.

Interview preparation tip

When asked to find records that don't match a condition, always prefer NOT EXISTS over NOT IN. It is safer regarding null values and often optimized better by the database engine.