HardCTEs High Frequency

30-Day Hospital Readmission Rate Benchmark

Frequently Asked By:AppleGoogleAmazon

Interactive In-Browser Workspace

Practice this problem on live schema tables in PostgreSQL 16 WASM with instant row verification and visual EXPLAIN plans.

Solve in Workbench

Problem Statement

Using a CTE, find all patients in `admissions_record` who were discharged and subsequently readmitted within 30 days. Calculate: 1. `total_discharges`: Count of all discharge events 2. `readmissions_within_30d`: Count of readmissions occurring within 30 days of previous discharge 3. `readmission_rate_pct`: `ROUND(100.0 * readmissions_within_30d / total_discharges, 2)` Return these three values in a single summary row.

Why Technical Interviewers Ask This

Federal health authorities track unplanned 30-day hospital readmissions. Clinical analysts use CTEs to measure what fraction of discharged patients were readmitted to the facility within 30 days.

Common Pitfalls & Interview Traps

Forgetting required sorting and deterministic tie-breaking.

Not accounting for NULL or zero-value denominators during calculations.

Using inefficient quadratic subqueries instead of single-pass ctes constructs.

Canonical Modern SQL Solution

PostgreSQL 16 / ANSI SQL

WITH pairs AS (
        SELECT 
          a1.stay_id,
          a1.patient_id,
          a1.discharge_date,
          CASE 
            WHEN EXISTS (
              SELECT 1 FROM admissions_record a2 
              WHERE a2.patient_id = a1.patient_id 
                AND a2.admission_date > a1.discharge_date 
                AND a2.admission_date <= a1.discharge_date + 30
            ) THEN 1 ELSE 0 
          END AS was_readmitted
        FROM admissions_record a1
      )
      SELECT 
        COUNT(*) AS total_discharges,
        SUM(was_readmitted) AS readmissions_within_30d,
        ROUND(100.0 * SUM(was_readmitted) / COUNT(*), 2) AS readmission_rate_pct
      FROM pairs;

Approach Breakdown

This query addresses the problem requirements by filtering and structuring data in CTEs. It meets the target criteria: Single row summary showing total discharges, readmissions within 30 days, and readmission rate percentage.

Edge Cases to Verbally Clarify with Your Interviewer

The dataset contains zero matching records (must return valid empty result without crashing).

Multiple records tie on sorting criteria (must evaluate tie-breaking order deterministically).

Foreign keys or secondary attributes have NULL values.

Frequently Asked Follow-Up Questions

What makes this question a favorite among Apple and Google interviewers?

Interviewers use this problem to test whether candidates write defensive, scalable CTEs queries that run efficiently on multi-million row production databases without excessive memory usage.

How does the query optimizer execute this query?

Using single-pass table scans with index lookups or sort buffers. Running EXPLAIN (ANALYZE, BUFFERS) in the DB Exercises workbench visualizes the execution nodes and buffer hit cache.