HardCTEs High Frequency

Surgical Suite Block Time Utilization Rate

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.

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Problem Statement

Join `operating_rooms` with `surgeries` using a CTE to aggregate utilized minutes. Calculate: 1. `room_name` 2. `allocated_minutes`: 480 (standard shift) 3. `utilized_minutes`: Sum of `duration_minutes` for surgeries conducted in that room 4. `utilization_pct`: `ROUND(100.0 * utilized_minutes / 480, 1)` Order results by `utilization_pct` descending.

Why Technical Interviewers Ask This

Hospital surgical leadership monitors surgical suite utilization. Surgical suites are scheduled in 480-minute blocks (8 hours). Efficiency audits compute what percentage of allocated block time was utilized for active surgery.

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 or_sums AS (
        SELECT 
          r.room_name,
          480 AS allocated_minutes,
          COALESCE(SUM(s.duration_minutes), 0) AS utilized_minutes
        FROM operating_rooms r
        LEFT JOIN surgeries s ON r.room_id = s.room_id
        GROUP BY r.room_id, r.room_name
      )
      SELECT 
        room_name,
        allocated_minutes,
        utilized_minutes,
        ROUND(100.0 * utilized_minutes / allocated_minutes, 1) AS utilization_pct
      FROM or_sums
      ORDER BY utilization_pct DESC;

Approach Breakdown

This query addresses the problem requirements by filtering and structuring data in CTEs. It meets the target criteria: Surgical rooms with total surgery minutes and utilization 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.