HardBusiness Analytics High Frequency

Clinical Pathology Antibiogram Susceptibility Matrix

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

Group culture sensitivity records in `antibiogram_isolates` by `pathogen_name`. Calculate: 1. `pathogen_name` 2. `total_isolates`: Total samples tested 3. `susceptible_count`: Count where `interpretation = 'Susceptible'` 4. `susceptibility_pct`: Percentage of isolates susceptible: `ROUND(100.0 * COUNT(*) FILTER (WHERE interpretation = 'Susceptible') / COUNT(*), 1)` Order results by `susceptibility_pct` descending, then by `pathogen_name` ascending.

Why Technical Interviewers Ask This

Hospital infection control committees construct an annual antibiogram matrix to guide empiric antimicrobial therapy: for each bacterial pathogen, compute the percentage of isolate samples tested that were susceptible.

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 business analytics constructs.

Canonical Modern SQL Solution

PostgreSQL 16 / ANSI SQL

SELECT 
        pathogen_name,
        COUNT(*) AS total_isolates,
        COUNT(*) FILTER (WHERE interpretation = 'Susceptible') AS susceptible_count,
        ROUND(100.0 * COUNT(*) FILTER (WHERE interpretation = 'Susceptible') / COUNT(*), 1) AS susceptibility_pct
      FROM antibiogram_isolates
      GROUP BY pathogen_name
      ORDER BY susceptibility_pct DESC, pathogen_name ASC;

Approach Breakdown

This query addresses the problem requirements by filtering and structuring data in Business Analytics. It meets the target criteria: Summary of pathogen isolates and susceptibility 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 Business Analytics 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.