Mongo

Basics

Command	Description
mongo	start the MongoDB shell
mongod	start the MongoDB server
mongos	start the MongoDB sharded cluster router

Database Operations

Operation	Description
use <database>	switch to a database
show dbs	show a list of databases
db.<collection>.insert()	insert a document into a collection
db.<collection>.find()	find documents in a collection
db.<collection>.update()	update documents in a collection
db.<collection>.remove()	remove documents from a collection
db.<collection>.aggregate()	perform aggregation operations on a collection

Query Operators

Operator	Description
$eq	matches values that are equal to a specified value
$ne	matches values that are not equal to a specified value
$gt	matches values that are greater than a specified value
$lt	matches values that are less than a specified value
$gte	matches values that are greater than or equal to a specified value
$lte	matches values that are less than or equal to a specified value
$in	matches any of the values specified in an array
$nin	matches none of the values specified in an array
$and	joins query clauses with a logical AND
$or	joins query clauses with a logical OR
$not	inverts the effect of a query expression
$exists	matches documents that have a specified field

Aggregation Pipeline Stages

Command	Description
$match	filters documents based on specified criteria
$group	groups documents by a specified field
$sort	sorts documents by a specified field
$skip	skips a specified number of documents in a collection
$limit	limits the number of documents returned from a collection
$project	specifies the fields to include or exclude from a document
$unwind	deconstructs an array field into individual documents
$lookup	performs a left outer join with another collection
$out	writes the results of an aggregation pipeline to a collection

Examples

Creating a database

use mydb;

Creating a collection

db.createCollection('students');

Inserting documents into a collection

db.students.insertOne({name: 'John Doe', age: 21, gender: 'M'});
db.students.insertMany([
  {name: 'Jane Doe', age: 19, gender: 'F'},
  {name: 'Bob Smith', age: 22, gender: 'M'}
]);

Updating documents in a collection

db.students.updateOne({name: 'John Doe'}, {$set: {age: 23}});
db.students.updateMany({gender: 'M'}, {$inc: {age: 1}});

Deleting documents from a collection

db.students.deleteOne({name: 'Bob Smith'});
db.students.deleteMany({gender: 'F'});

Finding documents in a collection

db.students.find();
db.students.find({gender: 'M'});
db.students.findOne({name: 'John Doe'});

Querying with operators

db.students.find({age: {$gt: 20}});
db.students.find({name: {$regex: /Doe/i}});

Sorting results

db.students.find().sort({age: -1});

Limiting results

db.students.find().limit(2);

Aggregating data

db.students.aggregate([
  {$group: {_id: '$gender', avg_age: {$avg: '$age'}}}
]);

Indexing collections for faster queries

db.students.createIndex({name: 1});

Joining collections with $lookup

db.students.aggregate([
  {
    $lookup: {
      from: 'grades',
      localField: '_id',
      foreignField: 'student_id',
      as: 'grades'
    }
  }
]);