Uptime Crew·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Apr 2026

Summary

SQL-heavy technical screen at Uptime Crew for a Software Engineer role. The main problem was a CASE statement categorization query with NULL handling, which sounds straightforward until the edge cases start piling up.

Questions Asked (1)

Write a SQL query that uses CASE statements to categorize rows based on multiple conditional rules, handles NULL values using COALESCE, NULLIF, or IS NULL checks, and produces derived columns suitable for downstream aggregation.

Data ModelingTechnical Trade-offsAlgorithms & Data Structures

Author's notes

The schema wasn't given upfront so I had to ask, which felt awkward but was probably the right move.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the business logic for categorization and the expected output schema, then walk through a concrete SQL query that uses CASE for multi-rule categorization, COALESCE/NULLIF/IS NULL for NULL handling, and aliases for derived columns. Emphasize how the derived columns enable downstream aggregation and discuss trade-offs like readability, performance, and maintainability.

Pro tip: Use COALESCE to provide default values for NULLs in the CASE conditions, but be cautious: COALESCE can mask data quality issues, so explicitly document assumptions. Also, consider using NULLIF to avoid division-by-zero or invalid comparisons in conditional logic.

1. Clarify requirements and edge cases

Ask about the categorization rules, the handling of NULLs, and the desired output columns. Confirm whether the query should be optimized for readability or performance.

2. Design the CASE logic

Outline the CASE statement with multiple WHEN conditions, ensuring order matters (e.g., most specific first). Use COALESCE or IS NULL checks within conditions to handle NULLs appropriately.

3. Write the SQL query

Construct the SELECT statement with CASE expressions, COALESCE/NULLIF/IS NULL as needed, and alias derived columns clearly. Include a FROM clause and any necessary JOINs or WHERE filters.

4. Validate and test

Mention testing with sample data, including NULLs and edge cases, to ensure the CASE logic works as intended. Discuss how to verify the derived columns produce correct aggregates.

5. Discuss trade-offs and optimizations

Talk about performance implications (e.g., CASE in WHERE vs. SELECT), readability, and maintainability. Suggest alternatives like using a lookup table if rules become complex.

Key Points to Mention

CASE statement syntax and ordering of WHEN conditions
COALESCE for default values and NULLIF for avoiding errors
IS NULL checks for explicit NULL handling
Aliasing derived columns for downstream use
Performance considerations: indexing, sargability, and avoiding functions on indexed columns
Readability and maintainability: commenting complex logic, using CTEs for clarity

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