SQL Query Debugging Quant Interview Guide
SQL query debugging quant interview guide for isolating CTEs, checking counts, inspecting joins, nulls, filters, examples, and fixes.
Candidates who need a reliable live debugging process.
Debug SQL one step at a time
When a SQL result is wrong, isolate the earliest step that changes row count, grain, or values unexpectedly. Random edits make debugging slower.
Counts are cheap evidence
Row counts before and after filters, joins, and aggregations show where data changed. They are often the fastest way to find a join or filter bug.
Concrete example
If total PnL doubles after a join, check whether the price table has multiple rows per symbol and date, causing trade rows to duplicate.
Inspect small samples
Look at a few representative keys, including missing and duplicate cases. Small examples make query behavior visible quickly.
Common mistakes
Candidates often stare at the final query. A better process checks each CTE or subquery at its intended grain and row count.
Practice the pattern
Use the LeetQuidity curriculum and calibration to turn this topic into a focused practice plan.