Guides · Technology

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.

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