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SQL Query Debugging Basics
Fix SQL queries methodically
This guide explains debugging SQL queries: simplify and run smaller parts, check filters and joins for row loss/duplication, verify aggregations, and compare row counts against expectations.
- sql debugging
- joins
- filters
- aggregation
- row counts
Reduce scope
Run the query without filters/joins to confirm base row counts.
Check joins
Look for unintended INNER joins dropping rows or wrong join keys causing duplication.
Validate filters
Add filters one at a time; watch for null-handling and date boundaries.
Verify aggregation
Check group-by columns and totals against known subtotals; watch for double-counting.