SQL Join and Completion Rates | Dataford Interview Questions

Your question is SQL Join and Completion Rates. Start with the requirements and the three tables on the right.

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Problem

Cengage wants a category-level view of learner progress across its digital learning products. Write a PostgreSQL query that joins users, enrollments, and courses to calculate enrollment and completion metrics by course category.

Requirements

Join the three tables using their key relationships.
Treat an enrollment as completed when completed_at is not NULL.
Return total enrollments, completed enrollments, and completion rate as a percentage, rounded to two decimal places.
Sort categories by completion rate descending, then category name ascending.

Schema

users

Column	Type	Description
user_idPK	INT	Unique learner identifier
full_name	VARCHAR(255)	Learner's full name
email	VARCHAR(255)	Learner email address

enrollments

Column	Type	Description
enrollment_idPK	INT	Unique enrollment identifier
user_id	INT	References users.user_id
course_id	INT	References courses.course_id
completed_at	TIMESTAMP	Timestamp when the learner completed the course

courses

Column	Type	Description
course_idPK	INT	Unique course identifier
course_name	VARCHAR(255)	Course title
category	VARCHAR(100)	Course subject category