Databases Lesson 8: MongoDB (NoSQL)

MongoDB stores data as JSON-like documents. No schema, no joins required — great for flexible data structures and rapid development.

Core Concepts

// SQL → MongoDB mapping:
// Database → Database
// Table → Collection
// Row → Document
// Column → Field
// JOIN → $lookup (or embed documents)

// A MongoDB document:
{
  _id: ObjectId("..."),
  name: "Alice",
  email: "alice@example.com",
  address: {               // embedded document
    city: "NYC",
    country: "USA"
  },
  tags: ["admin", "beta"],  // arrays natively!
  orders: [                 // embedded array
    { product: "Laptop", price: 999, date: ISODate("2024-01-15") }
  ]
}

CRUD Operations

// Insert
db.users.insertOne({ name: "Alice", age: 30 });
db.users.insertMany([{ name: "Bob" }, { name: "Charlie" }]);

// Find
db.users.find();                          // all
db.users.find({ age: { $gt: 25 } });     // where age > 25
db.users.findOne({ email: "a@b.com" });  // one document

// Update
db.users.updateOne({ email: "a@b.com" }, { $set: { age: 31 } });
db.users.updateMany({ role: "user" }, { $set: { active: true } });

// Delete
db.users.deleteOne({ _id: id });
db.users.deleteMany({ active: false });

Aggregation Pipeline

db.orders.aggregate([
  { $match: { status: "completed" } },        // filter
  { $group: { _id: "$userId", total: { $sum: "$amount" } } }, // group
  { $sort: { total: -1 } },                   // sort
  { $limit: 10 },                             // top 10
  { $lookup: {                                // join users collection
    from: "users", localField: "_id",
    foreignField: "_id", as: "user"
  } }
]);