SQL Conversion Rate Before/After | Dataford Interview Questions

Business Context

Disney+ France launched a new Watchlist feature and wants to evaluate whether it improved session conversion. A converted session is one where converted is TRUE.

Task

Write a PostgreSQL query that compares conversion performance during the 30 days before and the 30 days after the Watchlist launch on 2025-03-15.

Requirements

Join sessions to the Watchlist launch record and include only sessions in the two 30-day windows. The launch date belongs to the after period.
Return one row for each period with session count, converted session count, and conversion rate.
Treat NULL conversion flags as non-converted sessions, prevent division by zero, and order periods chronologically.

Schema

feature_launches

Column	Type	Description
launch_idPK	INT	Unique feature launch identifier
feature_name	VARCHAR(100)	Name of the Disney+ France feature
launch_date	DATE	Date when the feature launched

app_sessions

Column	Type	Description
session_idPK	INT	Unique viewing session identifier
user_id	INT	Viewer identifier
session_date	DATE	Date of the app session
converted	BOOLEAN	Whether the session resulted in conversion

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Problem