Getting Started

To use the MongoDB shell, you first need to install MongoDB on your system. After installation, you can start the shell by running the command:

mongosh

This will connect you to the default MongoDB instance running on your machine. If you need to connect to a specific instance, you can specify the hostname and port:

mongosh mongodb://<username>:<password>@hostname:port

Basic Operations

Connecting to a Database

To connect to a specific database, use the `use` command:

use myDatabase

Creating Collections

Collections are similar to tables in relational databases. You can create a collection using the `db.createCollection` method:

db.createCollection('myCollection')

Inserting Documents

Documents are the basic unit of data in MongoDB. You can insert documents using the `insertOne` or `insertMany` methods:

db.myCollection.insertOne({name: 'John Doe', age: 30})

db.myCollection.insertMany([ 
  {name: 'Jane Doe', age: 25}, 
  {name: 'Peter Pan', age: 12} 
])

Querying Documents

You can retrieve documents from a collection using the `find` method. You can specify query criteria using a JSON object:

db.myCollection.find({age: 30})

This query will retrieve all documents where the age is 30.

Updating Documents

You can update existing documents using the `updateOne` or `updateMany` methods:

db.myCollection.updateOne({name: 'John Doe'}, {$set: {age: 35}})

This will update the age of the document with the name 'John Doe' to 35.

Deleting Documents

You can delete documents from a collection using the `deleteOne` or `deleteMany` methods:

db.myCollection.deleteOne({name: 'John Doe'})

Advanced Operations

Aggregation Framework

The aggregation framework allows you to perform complex data transformations and aggregations. You can use various aggregation operators, such as `$group`, `$match`, `$project`, and more.

db.myCollection.aggregate([ 
  {$group: {_id: '$age', count: {$sum: 1}}}, 
  {$sort: {count: -1}} 
])

This aggregation will group documents by age and count the number of documents in each group, then sort the results by count in descending order.

MapReduce

MapReduce is a powerful tool for processing large amounts of data in MongoDB. It involves two phases: map and reduce. The map phase transforms documents into key-value pairs, and the reduce phase combines values with the same key.

db.myCollection.mapReduce( 
  function() { 
    emit(this.age, 1); 
  }, 
  function(key, values) { 
    return Array.sum(values); 
  }, 
  {out: 'ageCounts'} 
)

This MapReduce operation will create a new collection called 'ageCounts' with the count of documents for each age.

Useful Commands

• show dbs: Lists all available databases.
• show collections: Lists all collections in the current database.
• db.collection.find().pretty(): Displays documents in a more readable format.
• db.collection.count(): Counts the number of documents in a collection.
• db.collection.explain(): Provides information about query execution plans.

Security Considerations

Summary

• The MongoDB shell is a powerful tool for interacting with MongoDB databases.
• It provides a wide range of operations, from basic CRUD operations to advanced aggregation and MapReduce.
• You can connect to MongoDB instances, create collections, insert documents, query, update, delete, and perform various administrative tasks.
• It's important to prioritize security and use appropriate authentication mechanisms.