query-expert

# Query Expert

Master database queries across SQL and NoSQL systems. Generate optimized queries, analyze performance with EXPLAIN plans, design effective indexes, and troubleshoot slow queries.

## What This Skill Does

Helps you write efficient, performant database queries:
- **Generate Queries** - SQL, MongoDB, GraphQL queries
- **Optimize Queries** - Performance tuning and refactoring
- **Design Indexes** - Index strategies for faster queries
- **Analyze Performance** - EXPLAIN plans and query analysis
- **Troubleshoot** - Debug slow queries and bottlenecks
- **Best Practices** - Query patterns and anti-patterns

## Supported Databases

### SQL Databases
- **PostgreSQL** - Advanced features, CTEs, window functions
- **MySQL/MariaDB** - InnoDB optimization, replication
- **SQLite** - Embedded database optimization
- **SQL Server** - T-SQL, execution plans, DMVs
- **Oracle** - PL/SQL, partitioning, hints

### NoSQL Databases
- **MongoDB** - Aggregation pipelines, indexes
- **Redis** - Key-value queries, Lua scripts
- **Elasticsearch** - Full-text search queries
- **Cassandra** - CQL, partition keys

### Query Languages
- **SQL** - Standard and vendor-specific
- **MongoDB Query Language** - Find, aggregation
- **GraphQL** - Efficient data fetching
- **Cypher** - Neo4j graph queries

## SQL Query Patterns

### SELECT Queries

#### Basic SELECT

```sql
-- ✅ Select only needed columns
SELECT
    user_id,
    email,
    created_at
FROM users
WHERE status = 'active'
    AND created_at > NOW() - INTERVAL '30 days'
ORDER BY created_at DESC
LIMIT 100;

-- ❌ Avoid SELECT *
SELECT * FROM users;  -- Wastes resources
```

#### JOINs

```sql
-- INNER JOIN (most common)
SELECT
    o.order_id,
    o.total,
    c.name AS customer_name,
    c.email
FROM orders o
INNER JOIN customers c ON o.customer_id = c.customer_id
WHERE o.created_at >= '2024-01-01';

-- LEFT JOIN (include all left rows)
SELECT
    c.customer_id,
    c.name,
    COUNT(o.order_id) AS order_count,
    COALESCE(SUM(