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Blizzard Entertainment·Backend Engineer·Technical Phone Screen·Intermediate

Intermediate
Jul 2026

Summary

SQL round for a backend role at Blizzard, one problem centered on an auction-style schema. Nothing too wild but the LEFT JOIN requirement tripped me up a bit.

Questions Asked (1)

Q1

Given a lots table and a bids table, write a query that returns the winner (highest bidder) for each lot. Lots with no bids should still appear in the results, with a NULL winner.

Data ModelingAlgorithms & Data Structures
Author's notes

I jumped straight to GROUP BY and MAX(amount) and felt pretty good about it, then realized I'd written an INNER JOIN and was silently dropping all the empty lots.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the schema and edge cases (e.g., ties, NULL bids). Then use a LEFT JOIN from lots to bids and a correlated subquery or window function to find the highest bid per lot, ensuring lots with no bids return NULL. Finally, discuss performance and indexing.

Pro tip: Mention that you would add a tie-breaker (e.g., earliest bid time) to handle multiple highest bids, and use COALESCE to return NULL explicitly for no bids.

1. Clarify requirements and schema

Ask about table structures, data types, and whether ties are possible. Confirm that lots with no bids must appear with NULL winner.

2. Choose the right SQL technique

Decide between a correlated subquery, window function (ROW_NUMBER), or GROUP BY with MAX. Consider readability and performance.

3. Write the query with LEFT JOIN

Use LEFT JOIN from lots to bids to preserve all lots. Apply the chosen technique to select the highest bid per lot.

4. Handle ties and NULLs

If ties are possible, add a deterministic tie-breaker (e.g., earliest bid). Ensure NULL winner for lots with no bids.

5. Discuss performance and indexing

Mention indexes on bids(lot_id, amount) and that window functions may be more efficient than correlated subqueries for large data.

Key Points to Mention

  • LEFT JOIN ensures all lots appear, even without bids.
  • Use of window function ROW_NUMBER() OVER (PARTITION BY lot_id ORDER BY amount DESC) to rank bids.
  • Correlated subquery alternative: SELECT lot_id, (SELECT bidder FROM bids WHERE lot_id = lots.id ORDER BY amount DESC LIMIT 1) AS winner.
  • Handling ties: add a secondary sort key like bid_time ASC to pick a single winner.
  • NULL handling: COALESCE or explicit NULL when no bids exist.
  • Performance: index on bids(lot_id, amount DESC) to speed up ranking.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.