Explain the basic CRUD operations in MongoDB.

CRUD operations in MongoDB are the collection methods that create, read, update and delete documents. Each one has a form for one document and a form for many.

To create documents, insertOne() adds one document, and insertMany() takes an array and inserts each document in it. MongoDB adds an _id to any document that has none.

db.users.insertOne({name: "John", age: 30})
db.users.insertMany([
  {name: "Alice", age: 25},
  {name: "Bob", age: 35}
])

To read, find() returns a cursor over the matching documents, and with no filter it matches every document. findOne() returns the first match or null. A filter such as {age: {$gte: 25}} uses query operators.

db.users.find()
db.users.find({age: {$gte: 25}})
db.users.findOne({name: "John"})

To update, updateOne() changes the first match and updateMany() changes every match. Each takes a filter and an update document, and update operators such as $set change only the fields they name.

db.users.updateOne(
  {name: "John"}, 
  {$set: {age: 31}}
)
db.users.updateMany(
  {age: {$lt: 30}}, 
  {$set: {status: "young"}}
)

To delete, deleteOne() removes the first match and deleteMany() removes every match.

db.users.deleteOne({name: "John"})
db.users.deleteMany({age: {$lt: 18}})