sql-pro Checked

Optimizes SQL queries, designs database schemas, and troubleshoots performance issues. Use when a user asks why their query is slow, needs help writing complex joins or aggregations, mentions database performance issues, or wants to design or migrate a schema. Invoke for complex queries, window functions, CTEs, indexing strategies, query plan analysis, covering index creation, recursive queries, EXPLAIN/ANALYZE interpretation, before/after query benchmarking, or migrating queries between database dialects (PostgreSQL, MySQL, SQL Server, Oracle).

SQL Pro

Optimizes SQL queries, designs database schemas, and troubleshoots performance issues. Use when a user asks why their query is slow, needs help writing complex joins or aggregations, mentions database performance issues, or wants to design or migrate a schema. Invoke for complex queries, window functions, CTEs, indexing strategies, query plan analysis, covering index creation, recursive queries, EXPLAIN/ANALYZE interpretation, before/after query benchmarking, or migrating queries between database dialects (PostgreSQL, MySQL, SQL Server, Oracle).

SQL Pro

Core Workflow

1. **Schema Analysis** - Review database structure, indexes, query patterns, performance bottlenecks

2. **Design** - Create set-based operations using CTEs, window functions, appropriate joins

3. **Optimize** - Analyze execution plans, implement covering indexes, eliminate table scans

4. **Verify** - Run EXPLAIN ANALYZE and confirm no sequential scans on large tables; if query does not meet sub-100ms target, iterate on index selection or query rewrite before proceeding

5. **Document** - Provide query explanations, index rationale, performance metrics

Reference Guide

Load detailed guidance based on context:

| Topic | Reference | Load When |

|-------|-----------|-----------|

| Query Patterns | references/query-patterns.md | JOINs, CTEs, subqueries, recursive queries |

| Window Functions | references/window-functions.md | ROW_NUMBER, RANK, LAG/LEAD, analytics |

| Optimization | references/optimization.md | EXPLAIN plans, indexes, statistics, tuning |

| Database Design | references/database-design.md | Normalization, keys, constraints, schemas |

| Dialect Differences | references/dialect-differences.md | PostgreSQL vs MySQL vs SQL Server specifics |

Quick-Reference Examples

### CTE Pattern

-- Isolate expensive subquery logic for reuse and readability
WITH ranked_orders AS (
    SELECT
        customer_id,
        order_id,
        total_amount,
        ROW_NUMBER() OVER (PARTITION BY customer_id ORDER BY order_date DESC) AS rn
    FROM orders
    WHERE status = 'completed'          -- filter early, before the join
)
SELECT customer_id, order_id, total_amount
FROM ranked_orders
WHERE rn = 1;                           -- latest completed order per customer

### Window Function Pattern

-- Running total and rank within partition — no self-join required
SELECT
    department_id,
    employee_id,
    salary,
    SUM(salary)  OVER (PARTITION BY department_id ORDER BY hire_date) AS running_payroll,
    RANK()       OVER (PARTITION BY department_id ORDER BY salary DESC) AS salary_rank
FROM employees;