Explain MongoDB's data hierarchy: Database, Collection, Document.

MongoDB organizes data in a three-level hierarchy:

Database: Top-level container for collections

• Contains multiple collections
• Each database has its own files on disk
• Default databases: admin, local, config

Collection: Group of MongoDB documents

• Equivalent to a table in RDBMS
• Documents in a collection can have different schemas
• Collections are created automatically when first document is inserted

Document: Individual record in a collection

• JSON-like structure stored as BSON
• Can contain nested documents and arrays
• Maximum size: 16MB

Example structure:

Database: ecommerce
├── Collection: users
│   ├── Document: {_id: 1, name: "John", email: "john@email.com"}
│   └── Document: {_id: 2, name: "Jane", age: 25, city: "NYC"}
└── Collection: products
    └── Document: {_id: 1, title: "Laptop", price: 999.99}