Amazon·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

SQL-heavy technical screen for a BI Engineer role at Amazon. One question, but it had enough layers to keep me busy for a while.

Questions Asked (1)

Write a SQL query using a LEFT JOIN across multiple tables that includes groups with zero rows, using either a subquery in the SELECT/WHERE clause or a derived table in the FROM clause. Be prepared to explain NULL handling, performance considerations, and how you'd rewrite it using a CTE.

Data ModelingTechnical Trade-offsAlgorithms & Data Structures

Author's notes

The core join wasn't bad but the NULL handling follow-up tripped me up a bit.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the schema and the goal: you need all groups, even those with no related rows. Then write a query that uses a LEFT JOIN from the groups table to the detail tables, optionally with a subquery or derived table to pre-aggregate or filter. Finally, explain NULL handling, performance implications, and how a CTE can improve readability and maintainability.

Pro tip: Mention that you would test with a group that has zero rows to ensure the LEFT JOIN preserves it, and that you'd check the execution plan to confirm the join order and index usage.

1. Clarify requirements and schema

Ask about the tables involved, their relationships, and what 'groups with zero rows' means (e.g., groups with no orders). Confirm the desired output columns.

2. Write the core LEFT JOIN query

Use LEFT JOIN from the groups table to the detail tables, ensuring that groups with no matches are included. Optionally use a subquery in SELECT or a derived table in FROM to aggregate or filter before joining.

3. Explain NULL handling

Describe how NULLs appear for non-matching rows and how to handle them (e.g., COALESCE to replace NULL with 0). Warn about pitfalls like filtering on the right table's columns in WHERE, which turns the LEFT JOIN into an INNER JOIN.

4. Discuss performance considerations

Talk about indexing join keys, the cost of subqueries vs. joins, and how derived tables may be materialized. Mention that LEFT JOINs can be less efficient than INNER JOINs but are necessary for preserving groups.

5. Rewrite using a CTE

Show how to refactor the query with a CTE for better readability, especially if the subquery is complex. Explain that CTEs don't inherently improve performance but can help with maintenance and debugging.

Key Points to Mention

LEFT JOIN preserves all rows from the left table, filling NULLs for non-matching right table columns.
Using a subquery in SELECT can cause performance issues if not correlated properly; a derived table in FROM can pre-aggregate data.
Filtering on the right table's columns in the WHERE clause negates the LEFT JOIN; use ON clause or filter in a subquery instead.
COALESCE or IFNULL can replace NULLs with default values like 0 for counts.
CTEs improve readability and can be reused, but may not always optimize performance; check execution plans.
Indexes on join columns are crucial for performance; consider covering indexes for the columns selected.

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