Data Pipeline Reliability And Stream Processing

What's being tested

These problems test practical SQL/Python data-manipulation skills: joining event and reference tables, computing per-entity metrics, and using window functions to handle ordering, deduplication and week-over-week changes. Interviewers probe whether you can produce correct, auditable metrics from messy time-series data and explain tradeoffs for edge cases.

Patterns & templates

• ROW_NUMBER() over partitions for deduping: ROW_NUMBER() OVER (PARTITION BY id ORDER BY ts DESC) to keep the latest record.

• Pair in/out events with LEAD()/LAG() — compute durations as lead(ts)-ts, filter nulls and negative spans.

• Split cross-midnight shifts by truncating and using GREATEST()/LEAST() or generate_series() for per-day allocation.

• Aggregate defects: SUM(CASE WHEN condition THEN 1 ELSE 0 END) and defect rate = defects/NULLIF(total,0).

• Week-over-week: use DATE_TRUNC('week', dt) then SUM(...) + LAG() window to compute changes and percent deltas.

• Use GROUP BY + ORDER BY with ROW_NUMBER() to extract top-N lanes/carriers per metric; tie-break on stable key (e.g., carrier_id).

Common pitfalls

Pitfall: Pairing events by simple self-join without ordering — leads to wrong durations when multiple ins or outs exist. Always order and dedupe first.

Pitfall: Dividing by zero or using integer division — wrap denominators with NULLIF and cast to numeric to avoid truncation.

Pitfall: Ignoring late-arriving or duplicate events — surface a data-quality metric (e.g., percent unmatched events) instead of assuming perfect data.

Practice these

the practice cards below cover the canonical variants — solve all of them and time yourself