Running it first before reading it carefully was probably the wrong call.
Start by reading the query aloud and breaking it into logical clauses (SELECT, FROM, JOIN, WHERE, GROUP BY). Validate each clause against the schema and expected result set, fixing errors in a logical order (syntax first, then join logic, then column references). Explain each fix in terms of correctness, performance, and data integrity.
Pro tip: Before diving into fixes, ask clarifying questions about the schema and expected output—this shows you think about requirements first. Also, mention that you'd run the query after each fix to isolate errors, demonstrating a systematic debugging approach.
Check for basic syntax errors like missing commas, misspelled keywords, or incorrect clause order. Fix these first to ensure the query can execute.
Cross-check all column names against the actual schema to catch hallucinated or missing columns. Ensure all referenced columns exist and are spelled correctly.
Examine each JOIN to ensure the correct type (INNER, LEFT, etc.) is used based on the desired result set. Remove redundant joins that don't affect the output or cause duplication.
Look for joins that multiply rows unintentionally (fan-out) and ensure GROUP BY or DISTINCT is used appropriately. Verify that aggregate functions are applied correctly.
For each fix, articulate why it's necessary and any performance or correctness trade-offs. Discuss alternative approaches if applicable.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.