MySQL-relational database in general uses the JOIN operation. This allows you to perform queries across multiple tables.
MongoDB does not support joins but it does multi-dimensional data types such as arrays and even other documents. Placing one document inside another is referred to as embedding.
For example : If you want to create a blog using MySQL, you would have a table for posts and a table for comments. In MongoDB you might have a single collection of posts and an array of comments within each post.