// 连接到数据库
const MongoClient = require('mongodb').MongoClient;
const url = 'mongodb://localhost:27017';
const dbName = 'myDatabase';

MongoClient.connect(url, function(err, client) {
  const db = client.db(dbName);
  const collection = db.collection('myCollection');
  
  // 查询所有文档
  collection.find({}).toArray(function(err, docs) {
    console.log(docs);
    client.close();
  });
  
  // 带条件查询
  collection.find({ name: 'John' }).toArray(function(err, docs) {
    console.log(docs);
    client.close();
  });
});

collection.findOne({ age: { $gt: 30 } }, function(err, doc) {
  console.log(doc);
  client.close();
});

// 只获取name和age字段
collection.find({}, { projection: { name: 1, age: 1 } }).toArray(function(err, docs) {
  console.log(docs);
  client.close();
});

// 排除_id字段
collection.find({}, { projection: { _id: 0, name: 1 } }).toArray(function(err, docs) {
  console.log(docs);
  client.close();
});

collection.aggregate([
  { $match: { status: 'active' } },
  { $group: { _id: '$department', total: { $sum: '$salary' } } }
]).toArray(function(err, result) {
  console.log(result);
  client.close();
});

// 假设文档结构为 { user: { name: 'John', address: { city: 'NY' } } }
collection.find({}, { projection: { 'user.name': 1, 'user.address.city': 1 } })
  .toArray(function(err, docs) {
    console.log(docs);
  });

// 创建索引
collection.createIndex({ name: 1 });

// 限制返回数量
collection.find({}).limit(100).toArray(function(err, docs) {
  console.log(docs);
});