sql-correctness-reviewlisted

# SQL Correctness Review ## When to use this skill Use when a query might return **wrong results** — regardless of how fast it runs. Triggers: - "These numbers look too high/low" - "Revenue is double-counting" - "Why do I have duplicate rows?" - "This join is exploding" - "Check the logic of this query" - Two reports disagree on the same metric Routing: performance/anti-patterns → `sql-query-review`. Bad data *in the table* (not the query) → `data-quality-audit`. The first job here is deciding which of the two it is. ## Required inputs | Input | Why it matters | |---|---| | Query text | The suspect | | Expected output grain | "One row per X" — every check is relative to this | | Key relationships | Which joins are 1:1 vs 1:N (or what you *believe* they are) | | A reconciliation anchor | A number you trust (row count, total from a source system) to diff against | | Access to run SQL | Evidence queries are the core of this skill | ## The five bug classes | # | Class | Symptom | Canonical cause | |---|---|---|---| | 1 | Duplicates | Row count too high; metrics inflated | Source table grain misunderstood; missing dedup | | 2 | Join fanout | Totals inflate after a join | Joining on a partial key (1:N or N:M treated as 1:1) | | 3 | Wrong join type | Rows silently disappear | INNER where LEFT intended; WHERE on a LEFT join's right table | | 4 | NULL handling | Rows vanish or comparisons silently fail | NOT IN + NULL; `col != 'x'` dropping NULLs; NULL join keys | | 5 | CASE i