Top Clinical Sites by Enrollment | Dataford Interview Questions

Write a query to find the top three highest-performing clinical sites based on patient enrollment numbers.

Use clinical_sites and patient_enrollments. Count distinct non-null patients for each site, include only the three sites with the highest counts, and resolve ties by the smaller site_id.

Output

One row per selected clinical site.
Columns: site_id, site_name, and enrolled_patients.
Order by enrolled patient count descending, then site_id ascending.

Schema

clinical_sites

Column	Type	Description
site_idPK	INT	Unique clinical site identifier
site_name	VARCHAR(150)	Clinical site name

patient_enrollments

Column	Type	Description
enrollment_idPK	INT	Unique enrollment record identifier
site_id	INT	Clinical site associated with the enrollment
patient_id	INT	Patient identifier; duplicate records may exist
enrolled_at	DATE	Date of the enrollment record

Your question is Top Clinical Sites by Enrollment. 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