Query Builder Skill

Interactive database query builder for generating optimized SQL and NoSQL queries.

Instructions

You are a database query expert. When invoked:

1. Understand Requirements:

   • Analyze the requested data operations
   • Identify tables/collections and relationships
   • Determine filters, joins, and aggregations needed
   • Consider performance implications

2. Detect Database Type:

   • PostgreSQL, MySQL, SQLite (SQL databases)
   • MongoDB, DynamoDB (NoSQL databases)
   • Check for ORM usage (Prisma, TypeORM, SQLAlchemy, Mongoose)

3. Generate Queries:

   • Write optimized, readable queries
   • Use appropriate indexes and query patterns
   • Include parameterized queries to prevent SQL injection
   • Provide both raw SQL and ORM versions when applicable

4. Explain Query:

   • Break down query execution flow
   • Highlight performance considerations
   • Suggest indexes if needed
   • Provide alternative approaches when relevant

Supported Databases

• SQL: PostgreSQL, MySQL, MariaDB, SQLite, SQL Server
• NoSQL: MongoDB, DynamoDB, Redis, Cassandra
• ORMs: Prisma, TypeORM, Sequelize, SQLAlchemy, Django ORM, Mongoose

Usage Examples

@query-builder Get all users with their orders
@query-builder Find top 10 products by revenue
@query-builder --optimize SELECT * FROM users WHERE email LIKE '%@gmail.com'
@query-builder --explain-plan

SQL Query Patterns

Basic SELECT with Filters

-- PostgreSQL/MySQL
SELECT
  id,
  username,
  email,
  created_at
FROM users
WHERE
  active = true
  AND created_at >= NOW() - INTERVAL '30 days'
ORDER BY created_at DESC
LIMIT 100;

-- With parameters (prevent SQL injection)
SELECT * FROM users
WHERE email = $1 AND active = $2;

JOIN Operations

-- INNER JOIN - Get users with their orders
SELECT
  u.id,
  u.username,
  u.email,
  o.id as order_id,
  o.total_amount,
  o.created_at as order_date
FROM users u
INNER JOIN orders o ON u.id = o.user_id
WHERE o.status = 'completed'
ORDER BY o.created_at DESC;

-- LEFT JOIN - Include users without orders
SELECT
  u.id,
  u.username,
  COUNT(o.id) as order_count,
  COALESCE(SUM(o.total_amount), 0) as total_spent
FROM users u
LEFT JOIN orders o ON u.id = o.user_id
GROUP BY u.id, u.username
HAVING COUNT(o.id) > 0
ORDER BY total_spent DESC;

-- Multiple JOINs
SELECT
  o.id as order_id,
  u.username,
  p.name as product_name,
  oi.quantity,
  oi.price
FROM orders o
INNER JOIN users u ON o.user_id = u.id
INNER JOIN order_items oi ON o.id = oi.order_id
INNER JOIN products p ON oi.product_id = p.id
WHERE o.created_at >= '2024-01-01';

Aggregations

-- Group by with aggregations
SELECT
  DATE_TRUNC('day', created_at) as date,
  COUNT(*) as order_count,
  SUM(total_amount) as daily_revenue,
  AVG(total_amount) as avg_order_value,
  MAX(total_amount) as largest_order
FROM orders
WHERE created_at >= CURRENT_DATE - INTERVAL '7 days'
GROUP BY DATE_TRUNC('day', created_at)
ORDER BY date DESC;

-- Window functions
SELECT
  id,
  user_id,
  total_amount,
  created_at,
  ROW_NUMBER() OVER (PARTITION BY user_id ORDER BY created_at DESC) as order_rank,
  AVG(total_amount) OVER (PARTITION BY user_id) as user_avg_order
FROM orders;

Subqueries

-- Subquery in WHERE clause
SELECT * FROM users
WHERE id IN (
  SELECT DISTINCT user_id
  FROM orders
  WHERE total_amount > 1000
);

-- Subquery in SELECT (scalar subquery)
SELECT
  id,
  username,
  (SELECT COUNT(*) FROM orders WHERE user_id = users.id) as order_count,
  (SELECT MAX(total_amount) FROM orders WHERE user_id = users.id) as max_order
FROM users;

-- Common Table Expression (CTE)
WITH recent_orders AS (
  SELECT
    user_id,
    COUNT(*) as order_count,
    SUM(total_amount) as total_spent
  FROM orders
  WHERE created_at >= CURRENT_DATE - INTERVAL '30 days'
  GROUP BY user_id
)
SELECT
  u.id,
  u.username,
  u.email,
  COALESCE(ro.order_count, 0) as recent_orders,
  COALESCE(ro.total_spent, 0) as recent_spending
FROM users u
LEFT JOIN recent_orders ro ON u.id = ro.user_id
WHERE u.active = true;

Complex Queries

-- Recursive CTE for hierarchical data
WITH RECURSIVE category_tree AS (
  -- Base case: root categories
  SELECT id, name, parent_id, 0 as level
  FROM categories
  WHERE parent_id IS NULL

  UNION ALL

  -- Recursive case: child categories
  SELECT c.id, c.name, c.parent_id, ct.level + 1
  FROM categories c
  INNER JOIN category_tree ct ON c.parent_id = ct.id
)
SELECT * FROM category_tree
ORDER BY level, name;

-- Find top N per group
WITH ranked_products AS (
  SELECT
    p.*,
    c.name as category_name,
    ROW_NUMBER() OVER (PARTITION BY p.category_id ORDER BY p.sales DESC) as rank
  FROM products p
  INNER JOIN categories c ON p.category_id = c.id
)
SELECT * FROM ranked_products
WHERE rank <= 3;

UPSERT (INSERT or UPDATE)

-- PostgreSQL - ON CONFLICT
INSERT INTO users (id, username, email, updated_at)
VALUES ($1, $2, $3, NOW())
ON CONFLICT (id)
DO UPDATE SET
  username = EXCLUDED.username,
  email = EXCLUDED.email,
  updated_at = NOW();

-- MySQL - ON DUPLICATE KEY UPDATE
INSERT INTO users (id, username, email, updated_at)
VALUES (?, ?, ?, NOW())
ON DUPLICATE KEY UPDATE
  username = VALUES(username),
  email = VALUES(email),
  updated_at = NOW();

ORM Query Examples

Prisma (TypeScript)

// Basic query
const users = await prisma.user.findMany({
  where: {
    active: true,
    createdAt: {
      gte: new Date(Date.now() - 30 * 24 * 60 * 60 * 1000)
    }
  },
  orderBy: { createdAt: 'desc' },
  take: 100
});

// Relations
const userWithOrders = await prisma.user.findUnique({
  where: { id: userId },
  include: {
    orders: {
      where: { status: 'completed' },
      include: {
        items: {
          include: { product: true }
        }
      }
    }
  }
});

// Aggregations
const stats = await prisma.order.groupBy({
  by: ['userId'],
  where: {
    createdAt: {
      gte: new Date('2024-01-01')
    }
  },
  _count: { id: true },
  _sum: { totalAmount: true },
  _avg: { totalAmount: true }
});

// Raw SQL when needed
const result = await prisma.$queryRaw`
  SELECT * FROM users
  WHERE email = ${email}
  AND active = true
`;

TypeORM (TypeScript)

// Query builder
const users = await dataSource
  .getRepository(User)
  .createQueryBuilder('user')
  .where('user.active = :active', { active: true })
  .andWhere('user.createdAt >= :date', {
    date: new Date(Date.now() - 30 * 24 * 60 * 60 * 1000)
  })
  .orderBy('user.createdAt', 'DESC')
  .take(100)
  .getMany();

// Relations
const userWithOrders = await dataSource
  .getRepository(User)
  .createQueryBuilder('user')
  .leftJoinAndSelect('user.orders', 'order')
  .leftJoinAndSelect('order.items', 'item')
  .leftJoinAndSelect('item.product', 'product')
  .where('user.id = :id', { id: userId })
  .andWhere('order.status = :status', { status: 'completed' })
  .getOne();

// Aggregations
const stats = await dataSource
  .getRepository(Order)
  .createQueryBuilder('order')
  .select('order.userId', 'userId')
  .addSelect('COUNT(order.id)', 'orderCount')
  .addSelect('SUM(order.totalAmount)', 'totalSpent')
  .addSelect('AVG(order.totalAmount)', 'avgOrder')
  .where('order.createdAt >= :date', { date: new Date('2024-01-01') })
  .groupBy('order.userId')
  .getRawMany();

SQLAlchemy (Python)

from sqlalchemy import select, func, and_, or_
from datetime import datetime, timedelta

# Basic query
stmt = (
    select(User)
    .where(
        and_(
            User.active == True,
            User.created_at >= datetime.now() - timedelta(days=30)
        )
    )
    .order_by(User.created_at.desc())
    .limit(100)
)
users = session.execute(stmt).scalars().all()

# Joins
stmt = (
    select(User, Order)
    .join(Order, User.id == Order.user_id)
    .where(Order.status == 'completed')
    .order_by(Order.created_at.desc())
)
results = session.execute(stmt).all()

# Aggregations
stmt = (
    select(
        func.date_trunc('day', Order.created_at).label('date'),
        func.count(Order.id).label('order_count'),
        func.sum(Order.total_amount).label('revenue'),
        func.avg(Order.total_amount).label('avg_order')
    )
    .where(Order.created_at >= datetime.now() - timedelta(days=7))
    .group_by(func.date_trunc('day', Order.created_at))
    .order_by('date desc')
)
stats = session.execute(stmt).all()

# Raw SQL when needed
result = session.execute(
    text("SELECT * FROM users WHERE email = :email"),
    {"email": email}
).fetchall()

NoSQL Query Examples

MongoDB

// Basic query
db.users.find({
  active: true,
  createdAt: { $gte: new Date(Date.now() - 30 * 24 * 60 * 60 * 1000) }
})
.sort({ createdAt: -1 })
.limit(100);

// Aggregation pipeline
db.orders.aggregate([
  {
    $match: {
      status: 'completed',
      createdAt: { $gte: new Date('2024-01-01') }
    }
  },
  {
    $group: {
      _id: '$userId',
      orderCount: { $sum: 1 },
      totalSpent: { $sum: '$totalAmount' },
      avgOrder: { $avg: '$totalAmount' }
    }
  },
  {
    $sort: { totalSpent: -1 }
  },
  {
    $limit: 10
  }
]);

// Lookup (join)
db.users.aggregate([
  {
    $lookup: {
      from: 'orders',
      localField: '_id',
      foreignField: 'userId',
      as: 'orders'
    }
  },
  {
    $match: { 'orders.0': { $exists: true } }
  },
  {
    $project: {
      username: 1,
      email: 1,
      orderCount: { $size: '$orders' }
    }
  }
]);

Mongoose (Node.js)

// Basic query
const users = await User.find({
  active: true,
  createdAt: { $gte: new Date(Date.now() - 30 * 24 * 60 * 60 * 1000) }
})
.sort({ createdAt: -1 })
.limit(100);

// Population (join)
const user = await User.findById(userId)
  .populate({
    path: 'orders',
    match: { status: 'completed' },
    populate: {
      path: 'items.product'
    }
  });

// Aggregation
const stats = await Order.aggregate([
  {
    $match: {
      createdAt: { $gte: new Date('2024-01-01') }
    }
  },
  {
    $group: {
      _id: {
        $dateToString: { format: '%Y-%m-%d', date: '$createdAt' }
      },
      orderCount: { $sum: 1 },
      revenue: { $sum: '$totalAmount' },
      avgOrder: { $avg: '$totalAmount' }
    }
  },
  { $sort: { _id: -1 } }
]);

Performance Optimization

Use Indexes

-- Create indexes for frequently queried columns
CREATE INDEX idx_users_email ON users(email);
CREATE INDEX idx_orders_user_id ON orders(user_id);
CREATE INDEX idx_orders_created_at ON orders(created_at);

-- Composite index for multiple columns
CREATE INDEX idx_orders_user_status ON orders(user_id, status);

-- Partial index (PostgreSQL)
CREATE INDEX idx_active_users ON users(email) WHERE active = true;

-- Index for full-text search (PostgreSQL)
CREATE INDEX idx_products_search ON products
USING GIN(to_tsvector('english', name || ' ' || description));

Query Optimization Tips

-- ❌ Bad - SELECT *
SELECT * FROM users WHERE id = 1;

-- ✓ Good - Select only needed columns
SELECT id, username, email FROM users WHERE id = 1;

-- ❌ Bad - Function on indexed column
SELECT * FROM users WHERE LOWER(email) = 'user@example.com';

-- ✓ Good - Store lowercase email or use functional index
SELECT * FROM users WHERE email = 'user@example.com';

-- ❌ Bad - OR conditions can't use index efficiently
SELECT * FROM orders WHERE user_id = 1 OR customer_email = 'user@example.com';

-- ✓ Good - Use UNION when appropriate
SELECT * FROM orders WHERE user_id = 1
UNION
SELECT * FROM orders WHERE customer_email = 'user@example.com';

-- ❌ Bad - NOT IN with subquery
SELECT * FROM users WHERE id NOT IN (SELECT user_id FROM banned_users);

-- ✓ Good - LEFT JOIN with NULL check
SELECT u.* FROM users u
LEFT JOIN banned_users bu ON u.id = bu.user_id
WHERE bu.user_id IS NULL;

Pagination

-- ❌ Bad - OFFSET gets slower with large offsets
SELECT * FROM users
ORDER BY created_at DESC
LIMIT 20 OFFSET 10000;

-- ✓ Good - Cursor-based pagination
SELECT * FROM users
WHERE created_at < '2024-01-01 12:00:00'
ORDER BY created_at DESC
LIMIT 20;

-- ✓ Better - Keyset pagination
SELECT * FROM users
WHERE (created_at, id) < ('2024-01-01 12:00:00', 12345)
ORDER BY created_at DESC, id DESC
LIMIT 20;

Common Patterns

Soft Deletes

-- Add deleted_at column
ALTER TABLE users ADD COLUMN deleted_at TIMESTAMP NULL;

-- "Delete" by setting timestamp
UPDATE users SET deleted_at = NOW() WHERE id = 1;

-- Query active records
SELECT * FROM users WHERE deleted_at IS NULL;

-- Create index for better performance
CREATE INDEX idx_users_deleted_at ON users(deleted_at)
WHERE deleted_at IS NULL;

Audit Trail

-- Audit table
CREATE TABLE audit_log (
  id SERIAL PRIMARY KEY,
  table_name VARCHAR(50),
  record_id INTEGER,
  action VARCHAR(10),
  old_values JSONB,
  new_values JSONB,
  changed_by INTEGER,
  changed_at TIMESTAMP DEFAULT NOW()
);

-- Trigger for automatic audit
CREATE OR REPLACE FUNCTION audit_trigger()
RETURNS TRIGGER AS $$
BEGIN
  INSERT INTO audit_log (table_name, record_id, action, old_values, new_values, changed_by)
  VALUES (
    TG_TABLE_NAME,
    NEW.id,
    TG_OP,
    row_to_json(OLD),
    row_to_json(NEW),
    current_user_id()
  );
  RETURN NEW;
END;
$$ LANGUAGE plpgsql;

Running Totals

-- Window function approach
SELECT
  date,
  daily_revenue,
  SUM(daily_revenue) OVER (ORDER BY date) as running_total
FROM daily_stats
ORDER BY date;

Anti-Patterns to Avoid

N+1 Query Problem

// ❌ Bad - N+1 queries
const users = await User.findAll();
for (const user of users) {
  const orders = await Order.findAll({ where: { userId: user.id } });
  // Process orders...
}

// ✓ Good - Single query with join
const users = await User.findAll({
  include: [{ model: Order }]
});

Missing Indexes

-- ❌ Bad - No index on foreign key
SELECT * FROM orders WHERE user_id = 123; -- Slow!

-- ✓ Good - Index on foreign key
CREATE INDEX idx_orders_user_id ON orders(user_id);

Retrieving Too Much Data

-- ❌ Bad - Fetching all rows
SELECT * FROM orders; -- Could be millions of rows!

-- ✓ Good - Use pagination
SELECT * FROM orders
ORDER BY created_at DESC
LIMIT 100;

Best Practices

1. Always use parameterized queries to prevent SQL injection
2. Index foreign keys and frequently queried columns
3. Use EXPLAIN ANALYZE to understand query performance
4. **Avoid SELECT *** - only fetch needed columns
5. Use transactions for data consistency
6. Implement pagination for large datasets
7. Cache frequently accessed data (Redis, Memcached)
8. Monitor slow queries and optimize them
9. Use connection pooling to manage database connections
10. Regular VACUUM and ANALYZE on PostgreSQL

Notes

• Test queries with realistic data volumes
• Monitor query execution time in production
• Use read replicas for read-heavy workloads
• Consider database-specific features (PostgreSQL extensions, MySQL storage engines)
• Document complex queries with comments
• Keep ORMs updated but know raw SQL for complex operations