Explain the basic CRUD operations in MongoDB.

Create:

// Insert one document
db.users.insertOne({name: "John", age: 30})

// Insert multiple documents
db.users.insertMany([
  {name: "Alice", age: 25},
  {name: "Bob", age: 35}
])

Read:

// Find all documents
db.users.find()

// Find with criteria
db.users.find({age: {$gte: 25}})

// Find one document
db.users.findOne({name: "John"})

Update:

// Update one document
db.users.updateOne(
  {name: "John"}, 
  {$set: {age: 31}}
)

// Update multiple documents
db.users.updateMany(
  {age: {$lt: 30}}, 
  {$set: {status: "young"}}
)

Delete:

// Delete one document
db.users.deleteOne({name: "John"})

// Delete multiple documents
db.users.deleteMany({age: {$lt: 18}})