MediumAggregations

Fleet Route Delivery SLA Adherence

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 deliveries in `courier_deliveries` by `delivery_zone`. Calculate: 1. `delivery_zone` 2. `total_packages`: Count of all deliveries 3. `avg_transit_minutes`: Average `transit_minutes` rounded to 1 decimal place 4. `on_time_pct`: Percentage of deliveries marked as on-time (`is_on_time = true`): `ROUND(100.0 * SUM(CASE WHEN is_on_time THEN 1 ELSE 0 END) / COUNT(*), 1)` Order results by `on_time_pct` ascending to see troubled zones first.

Why Technical Interviewers Ask This

QuickRide Logistics guarantees delivery windows across urban delivery zones. Regional fleet dispatchers need to audit delivery completion times and identify zones failing their 90% on-time target.

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 
        delivery_zone,
        COUNT(*) AS total_packages,
        ROUND(AVG(transit_minutes), 1) AS avg_transit_minutes,
        ROUND(100.0 * SUM(CASE WHEN is_on_time THEN 1 ELSE 0 END) / COUNT(*), 1) AS on_time_pct
      FROM courier_deliveries
      GROUP BY delivery_zone
      ORDER BY on_time_pct ASC;

Approach Breakdown

This query addresses the problem requirements by filtering and structuring data in Aggregations. It meets the target criteria: Delivery metrics per zone showing total packages, average transit time, and on-time 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 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.