SQL DISTINCT Query | Dataford Interview Questions

An innovative healthcare organization uses its Care Insights platform to review diagnoses recorded during patient encounters. Write a PostgreSQL query that returns distinct diagnosis codes and names for qualifying encounters.

Requirements

Join patients, encounters, and diagnoses.
Include only active patients with completed encounters on or after 2025-01-01.
Exclude diagnoses with a NULL code and return each diagnosis only once.
Sort the results by diagnosis code in ascending order.

Schema

patients

Column	Type	Description
patient_idPK	INT	Unique patient identifier
patient_name	VARCHAR(100)	Patient display name
status	VARCHAR(20)	Current patient status

encounters

Column	Type	Description
encounter_idPK	INT	Unique encounter identifier
patient_id	INT	Patient associated with the encounter
encounter_date	DATE	Date the encounter occurred
status	VARCHAR(20)	Encounter workflow status

diagnoses

Column	Type	Description
diagnosis_idPK	INT	Unique diagnosis record identifier
encounter_id	INT	Encounter associated with the diagnosis
diagnosis_code	VARCHAR(20)	Clinical diagnosis code
diagnosis_name	VARCHAR(150)	Human-readable diagnosis name

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