Amazon·Data Scientist·Technical Phone Screen·Intermediate

Intermediate

May 2024Remote

Summary

Amazon data scientist technical screen with a SQL-heavy question about VARCHAR primary keys and self-joins. Pretty niche topic, not your typical aggregation or window function stuff.

Questions Asked (1)

Given a visits table where the primary key is stored as VARCHAR but contains numeric strings, write a SQL query returning all ordered pairs of visits where v1.visit_id < v2.visit_id using lexicographic comparison. Then explain why this VARCHAR comparison can produce unexpected ordering (e.g. '9' vs '10'), and show how you'd enforce correct numeric ordering if needed.

Data ModelingTechnical Trade-offsProduct Analytics & Metrics

Author's notes

The self-join part came naturally enough, just FROM visits t1 JOIN visits t2 ON t1.visit_id < t2.visit_id.

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AI HintsAI Generated

Suggested Approach

Start by writing the self-join query with the lexicographic condition, then clearly explain why VARCHAR comparison causes '9' > '10', and finally demonstrate how to enforce numeric ordering using CAST or by changing the column type. Emphasize the importance of data types in analytics and the trade-offs between flexibility and correctness.

Pro tip: Mention that while casting in the query works, the best long-term fix is to store numeric IDs as integers or use a computed column with an index to avoid performance hits and ensure consistent ordering.

1. Write the lexicographic self-join query

Use a self-join on the visits table with the condition v1.visit_id < v2.visit_id to return all ordered pairs. Ensure you select both visit IDs for clarity.

2. Explain lexicographic comparison pitfalls

Describe how string comparison works character by character, leading to '9' being greater than '10' because '9' > '1'. Highlight that this can cause incorrect ordering in reports and analyses.

3. Show how to enforce numeric ordering

Demonstrate casting the VARCHAR to an integer (e.g., CAST(v1.visit_id AS INTEGER) < CAST(v2.visit_id AS INTEGER)) or using a numeric column if available. Discuss the performance implications of casting in the join condition.

4. Discuss trade-offs and best practices

Talk about the trade-offs between storing IDs as strings (flexibility, leading zeros) versus integers (correct ordering, performance). Recommend schema changes or computed columns for production systems.

Key Points to Mention

Lexicographic vs numeric comparison: how string sorting works character by character.
Example: '9' vs '10' in string comparison yields '9' > '10'.
Self-join syntax and the importance of aliasing.
Casting to integer for correct ordering, but beware of performance and index usage.
Best practice: store numeric identifiers as numeric types; if strings are necessary, consider zero-padding or a separate numeric column.
Impact on analytics: incorrect ordering can lead to wrong aggregations, trends, and insights.

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