Use when reviewing a SQL query before it runs in production — catches accidental fan-out joins, non-idempotent MERGE on an incomplete key, missing partition filters on large tables, and NULL-in-aggregate bugs that silently change the result. Triggers on "review this query", "is this SQL safe to run", "check my MERGE statement", "why is this join returning too many rows".
Installs into .claude/skills of the current project.
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---
name: sql-review
description: Use when reviewing a SQL query before it runs in production — catches accidental fan-out joins, non-idempotent MERGE on an incomplete key, missing partition filters on large tables, and NULL-in-aggregate bugs that silently change the result. Triggers on "review this query", "is this SQL safe to run", "check my MERGE statement", "why is this join returning too many rows".
---
# SQL Review
Before a query ships, check it against the four silent-wrong-answer traps and, for
each, name the specific risk in THIS query and the fix:
1. **Fan-out join** — a join key that is not unique on the right side multiplies rows.
2. **Non-idempotent MERGE** — a MERGE keyed on an incomplete grain double-applies on
re-run.
3. **Missing partition filter** — a scan over an unpartitioned date range is slow and
expensive.
4. **NULL in aggregate** — COUNT/SUM silently skip NULLs, changing the result vs intent.
