MediumAggregations

Hospital Ward Inpatient Length of Stay & Bed Occupancy

Frequently Asked By:AppleGoogleAmazon

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

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Problem Statement

Aggregate patient stays in the `ward_admissions` table grouped by `department_name`. Calculate: 1. `department_name` 2. `total_admissions`: Count of all admissions 3. `avg_stay_days`: Average of `stay_duration_days` rounded to 1 decimal place 4. `max_stay_days`: Maximum `stay_duration_days` in that department Only include departments that have **at least 2** admissions. Order results by `avg_stay_days` descending.

Why Technical Interviewers Ask This

Hospital bed administration needs to optimize ward resource allocation. Operations wants a breakdown of inpatient stays grouped by medical department to identify wards with high patient turnover vs extended average length of stay (ALOS).

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 aggregations constructs.

Canonical Modern SQL Solution

PostgreSQL 16 / ANSI SQL

SELECT 
        department_name,
        COUNT(*) AS total_admissions,
        ROUND(AVG(stay_duration_days), 1) AS avg_stay_days,
        MAX(stay_duration_days) AS max_stay_days
      FROM ward_admissions
      GROUP BY department_name
      HAVING COUNT(*) >= 2
      ORDER BY avg_stay_days DESC;

Approach Breakdown

This query addresses the problem requirements by filtering and structuring data in Aggregations. It meets the target criteria: Summary of department_name, total_admissions, avg_stay_days, and max_stay_days for wards with >= 2 admissions.

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 Aggregations 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.