MongoDB

MongoDB Basics

Command	Description
mongod	Start MongoDB server
mongo	Open the MongoDB shell
show dbs	Show all the available databases
use db_name	Switch to the specified database or create one if it doesn't exist
db	Display the current database you are using
db.dropDatabase()	Drop the current database

CRUD Operations

Insert

Command	Description
db.collection.insertOne(document)	Insert a new document into the collection
db.collection.insertMany([document1, document2, ...])	Insert multiple documents into the collection

Find

Command	Description
db.collection.find()	Retrieve all documents from the collection
db.collection.find(query)	Retrieve documents that match the specified query
db.collection.findOne()	Retrieve one document from the collection
db.collection.findOne(query)	Retrieve one document that matches the specified query

Update

Command	Description
db.collection.updateOne(query, update)	Update the first document that matches the query
db.collection.updateMany(query, update)	Update all documents that match the query
db.collection.replaceOne(query, replacement)	Replace the first document that matches the query

Delete

Command	Description
db.collection.deleteOne(query)	Delete the first document that matches the query
db.collection.deleteMany(query)	Delete all documents that match the query

Query Operators

Comparison Operators

Operator	Description	Example
$eq	Matches values that are equal to a specified value	{ age: { $eq: 18 } }
$ne	Matches all values that are not equal to a specified value	{ age: { $ne: 18 } }
$gt	Matches values that are greater than a specified value	{ age: { $gt: 18 } }
$gte	Matches values that are greater than or equal to a specified value	{ age: { $gte: 18 } }
$lt	Matches values that are less than a specified value	{ age: { $lt: 18 } }
$lte	Matches values that are less than or equal to a specified value	{ age: { $lte: 18 } }
$in	Matches any of the values specified in an array	{ name: { $in: ['Alice', 'Bob'] } }
$nin	Matches none of the values specified in an array	{ name: { $nin: ['Alice', 'Bob'] } }

Logical Operators

Operator	Description	Example
$and	Joins query clauses with a logical AND	{ $and: [ { name: 'Alice' }, { age: 18 } ] }
$or	Joins query clauses with a logical OR	{ $or: [ { name: 'Alice' }, { name: 'Bob' } ] }
$not	Inverts the effect of a query expression	{ age: { $not: { $gte: 18 } } }
$nor	Joins query clauses with a logical NOR	{ $nor: [ { name: 'Alice' }, { name: 'Bob' } ] }

Element Operators

Operator	Description	Example
$exists	Matches documents that have the specified field	{ age: { $exists: true } }

Create collection in MongoDB

To create a collection in MongoDB, you can use the db.createCollection() method in the MongoDB shell. The syntax for creating a collection is as follows:

db.createCollection("<collection_name>", <options>)

Where <collection_name> is the name of the collection you want to create and <options> are the optional parameters that you can use to configure the collection. Here are some examples of how you can create a collection:

1. Create a collection without any options:

> db.createCollection("users")

This will create a collection named "users" in the current database.

2. Create a collection with options:

> db.createCollection("users", {
    capped: true,
    size: 100000,
    max: 100
  })

This will create a capped collection named "users" with a maximum size of 100,000 bytes and a maximum number of documents of 100.

You can also create a collection using the MongoCollection class in a MongoDB driver for your programming language. The specific syntax for doing this will depend on the driver you are using.