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SQL query explainer

Explains what a SQL query does in plain language, step by step, and flags performance and correctness risks. Use when the user pastes a SQL query and asks what it does, why it is slow, or whether it is correct.

Save these files in a folder named sql-query-explainer, then add that folder to your agent's skills.

SKILL.md

md
---
name: sql-query-explainer
description: "Explains what a SQL query does in plain language, step by step, and flags performance and correctness risks. Use when the user pastes a SQL query and asks what it does, why it is slow, or whether it is correct."
---

# SQL query explainer

## Steps

1. State in one sentence what the query returns, in business terms.
2. Walk through it in execution order: FROM/JOIN → WHERE → GROUP BY → HAVING → SELECT → ORDER BY → LIMIT.
3. For each join, say the join type and what happens to rows without a match.
4. Flag risks:
   - joins that can multiply rows (fan-out) before an aggregate
   - `NULL` handling in `WHERE`, `NOT IN` and outer joins
   - functions on indexed columns in `WHERE` that prevent index use
   - `SELECT *`, missing `LIMIT` on large tables, implicit type casts
5. If asked to optimize, propose one rewritten query and explain what changed. Ask for the dialect (PostgreSQL, MySQL, BigQuery…) when syntax differs.

## Rules

- Do not claim performance numbers without an `EXPLAIN` plan; suggest running one.
- Keep identifiers exactly as written.

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