CTE for Patients with Multiple Diagnoses | Dataford Interview Questions

Novo Nordisk clinical analytics needs to identify patients who received multiple distinct diagnoses during the same calendar month. Write a PostgreSQL query using a common table expression (CTE).

Requirements

Exclude diagnosis records with a missing diagnosis date or diagnosis code.
Group diagnoses by patient and calendar month, counting distinct diagnosis codes.
Return only patient-month groups with more than one distinct diagnosis.
Join patient details and include the diagnosis codes in alphabetical order.

Schema

patients

Column	Type	Description
patient_idPK	INT	Unique patient identifier
patient_name	VARCHAR(100)	Patient name

diagnoses

Column	Type	Description
diagnosis_idPK	INT	Unique diagnosis record identifier
patient_id	INT	Referenced patient identifier
diagnosed_at	DATE	Date on which the diagnosis was recorded
diagnosis_code	VARCHAR(50)	Diagnosis code or clinical diagnosis description

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