SQL databases store data in tables with a fixed schema and join tables at query time. NoSQL databases such as MongoDB drop the fixed schema and store data in other shapes, like documents, so related data can live together.
SQL databases
MongoDB
Data model
Tables of rows and columns
Collections of JSON-like documents
Schema
Fixed and defined in advance
Flexible, so documents can differ
Relationships
Joins across tables
Embedded data, or $lookup when needed
Scaling
Mostly vertical, on a larger server
Horizontal, through sharding, which splits data across many servers
Transactions
ACID transactions are standard
Single-document writes are atomic, and multi-document ACID transactions are available
Choose MongoDB when the data is semi-structured or changes shape often, and when documents match the objects in your code. That match speeds up development, which suits fast-moving projects such as modern web applications. MongoDB also suits applications that must scale horizontally.
A relational database stays the better fit when most queries join many tables. SQL databases also have a mature ecosystem of tools.
Rewriting in plainer words…
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