SQL Time Between Visits | Dataford Interview Questions

Your question is SQL Time Between Visits. Start with the requirements and the one table on the right.

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

Problem

Tempus Lens uses longitudinal patient records to support clinical insights. Given a patient visit table, write a PostgreSQL query that measures the interval between each patient's first and second completed visits.

Requirements

Consider only visits with visit_status = 'completed' and a non-null visit_started_at.
Number each patient's valid visits chronologically, using visit_id to break timestamp ties.
Return only patients with at least two valid completed visits, including the first visit timestamp, second visit timestamp, and elapsed interval.
Sort the final results by patient_id.

Schema

patient_visits

Column	Type	Description
visit_idPK	INT	Unique identifier for the visit
patient_id	INT	Identifier of the patient receiving care
visit_started_at	TIMESTAMP	Timestamp when the visit started
visit_status	VARCHAR(20)	Status or outcome of the visit

Your question is SQL Time Between Visits. Start with the requirements and the one table on the right.

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

Requirements

Consider only visits with visit_status = 'completed' and a non-null visit_started_at.
Number each patient's valid visits chronologically, using visit_id to break timestamp ties.
Return only patients with at least two valid completed visits, including the first visit timestamp, second visit timestamp, and elapsed interval.
Sort the final results by patient_id.

Schema

patient_visits

Column	Type	Description
visit_idPK	INT	Unique identifier for the visit
patient_id	INT	Identifier of the patient receiving care
visit_started_at	TIMESTAMP	Timestamp when the visit started
visit_status	VARCHAR(20)	Status or outcome of the visit