MediumAggregations

ATM Cash Depletion & Peak Withdrawal Volume

Frequently Asked By:StripeCoinbaseBloomberg

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 transactions in `atm_withdrawals` by `terminal_id` and `branch_location`. Calculate: 1. `terminal_id` 2. `branch_location` 3. `withdrawal_count`: Total count of withdrawal events 4. `total_cash_dispensed`: Sum of `dispensed_usd` 5. `avg_withdrawal_size`: `ROUND(AVG(dispensed_usd), 2)` Use `HAVING` to only include terminals where `total_cash_dispensed >= 20000.00` AND `withdrawal_count >= 3`. Order results by `total_cash_dispensed` descending.

Why Technical Interviewers Ask This

Bank vault logistics must route cash replenishment trucks before ATMs run dry over holiday weekends. The operations desk needs a list of ATM terminals where total cash dispensed today exceeds $20,000 across at least 3 transactions.

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 
        terminal_id,
        branch_location,
        COUNT(*) AS withdrawal_count,
        SUM(dispensed_usd) AS total_cash_dispensed,
        ROUND(AVG(dispensed_usd), 2) AS avg_withdrawal_size
      FROM atm_withdrawals
      GROUP BY terminal_id, branch_location
      HAVING SUM(dispensed_usd) >= 20000.00 AND COUNT(*) >= 3
      ORDER BY total_cash_dispensed DESC;

Approach Breakdown

This query addresses the problem requirements by filtering and structuring data in Aggregations. It meets the target criteria: High-volume ATMs requiring priority cash restocking, sorted by cash dispensed.

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 Stripe and Coinbase 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.