How to Perform the SQL Join Equivalent in MongoDB?

MongoDB, being a NoSQL database, is quite different from SQL databases. MongoDB is schema-less and document-oriented.
It means it stores data in flexible BSON documents instead of predefined tables.
As a result, the concept of joins in MongoDB doesn’t directly translate from SQL.
In MongoDB, achieving joins as SQL joins is possible through aggregation pipelines and the use of the $lookup operator.
Aggregation pipelines allow us to process data records and transform them using a series of stages.

Let’s set up an Environment:

To understand How to perform the SQL Join equivalent in MongoDB we need a collection and some documents on which we will perform various operations and queries. Here we will consider two collections called Books and Authors which contain various information shown in the below images.

Books Collection

Authors Collection:

Authors Collection

Below Synax used for performing Joins

db.modelName.aggregate([
    {
        $lookup: {
            from: "Collection to Join",
            localField: "Field from the input documents",
            foreignField: "Field from the documents of the 'from' collection",
            as: "Pick a field-name as output"
        }
    }
]);

Explanation: This MongoDB aggregation syntax utilizes the $lookup stage to perform a left join between two collections.

from: This represents the foreign collection, which we want to join to our current collection.
localField: LocalField specifies the field in the input documents to perform the join operation.
foreignField: The foreignField specifies the field from the foreign collection to match with the localField of the input documents.
as: It denotes the name of the field that will contain the join array data in the output documents.