# Introduction to MongoDB
MongoDB is a document-oriented NoSQL database used for high volume data storage.
NoSQL databases (aka "not only SQL") are usually non-tabular and store data differently than relational tables. They come in a variety of types based on their data model. The main types are document, key-value, wide-column, and graph.
# How is It Different from SQL Databases?
Relational databases like MySQL, PostgreSQL primarily use tables, rows, and columns to store data in structured schemas.
A database schema is the skeleton structure that represents the logical view of the entire database. It defines how to organize the data and dictates relationships among different data points. It also formulates all the constraints that are to be enforced on the data.
MongoDB doesn't have a strict schema; instead, it uses a document-based NoSQL data model.
# Key Components of MongoDB Architecture
If you're familiar with SQL databases, you can think of collections as tables and the documents as rows in the table.
The following diagram shows the anatomy of a MongoDB document:
Below are a few terms that appear frequently in the context of MongoDB:
_id – This is a field required in every MongoDB document. The _id field represents a unique value in the MongoDB document. It acts like the document's primary key. If you create a new document without an _id field, MongoDB will automatically create the field.
Collection – This is a grouping of MongoDB documents. A collection exists within a single database. As seen from the introduction, collections don't enforce any rigid schema. Different collections can house documents of arbitrary shape and size.
Cursor – This is a pointer to the result set of a query. Clients can iterate through a cursor to retrieve results.
Database – This is a container for collections like in relational databases wherein it is a container for tables. Each database gets its own set of files on the file system. A MongoDB server can store multiple databases.
Document - A record in a MongoDB collection is called a document. The document, in turn, will consist of field names and values.
Field - A key-value pair in a document. A document has zero or more fields. Fields are similar to columns in relational databases.
# Feature Sets
NoSQL databases like MongoDB offer many advantages over their SQL counterparts. Here's an overview of the features that aim to give you a glimpse of how MongoDB can make web development easier:
The size and contents of each document can be different from each other. This flexibility in the data model often leads to faster development cadence.
The documents don't need to have a schema defined beforehand. Instead, you can create the fields on the fly.
In relational databases, you usually have to perform queries to retrieve data and map those tabular data to your code's data structures like lists, classes, dictionaries manually. On the other hand, the JSON-like document model resembles how you construct classes and objects in Python and other similar programming languages. This means mapping data requires less code from your side.
The data model available within MongoDB allows you to represent hierarchical relationships, to store arrays, and other more complex structures easily.
Despite having many advantages, MongoDB also suffers from a few limitations. Let’s discuss some of them here:
MongoDB doesn’t support join like relational databases. You can join documents in your code (in Python) but it won't be as performant as native join-operations in relational databases.
MongoDB stores key names for each value pairs. Also, due to the lack of the join-operation functionality, there's data redundancy. This results in high memory usage.
MongoDB imposes a limit on your document size. Document size can't exceed 16MB.
You cannot have more than 100 levels of nesting in a single document.
In this section, you've got a basic overview of MongoDB and how NoSQL databases differ from their relational counterparts. Also, you've learned about the data model MongoDB uses and what features it offers that sets it apart from other databases.
In the next section, you'll learn how you can get up and running with MongoDB via MongoDB Compass and Atlas.