How does mongodb $group work?
Group operator (also known as an accumulator operator) is a crucial operator in the MongoDB language, as it helps to perform various transformations of data. It is a part of aggregation in MongoDB. MongoDB is an open-source NoSQL database management program. Show NoSQL is an alternative to traditional relational databases. NoSQL databases are quite useful when working with large sets of distributed data. MongoDB is a tool that can manage document-oriented information, store, or retrieve information. In this article, we are going to learn about the $group operator with some examples. PrerequisitesBefore we start make sure you have the following:
AggregationAggregation is an operation that processes data to give a computed result. Aggregation operations group values from multiple documents together and can perform a variety of operations on the grouped data to return a single result. It involves stages or pipelines through which data is processed to yield a combined result. Pipelines are stages through which data is processed, more technically transformed into by the provided criteria. Each pipeline is independent and receives the data of the previous stage. Always remember that the first pipeline will have direct contact with the data itself, not its transformed version, and it can take advantage of indexes. What is the $group operator?As the name suggests, the Suppose
there are 15 people in a database and they all have a similar hobby. If we want to count all those people who have a hobby in common, then the Let’s take a look at its syntax:
The The
The $group operator does not remove the data from the database. It just aggregates multiple documents into a single document based on the field passed to the Important points to remember
ExampleNow, let’s look at an example. The code snippets should be run in the mongo shell. Let’s create a database: Now, let’s create a collection named
Now that we have some documents in the collection, let’s use the To do that we have to use You have to provide a key that’ll hold the result of the accumulator operator. For this example, we are using Synatx of
Now applying the logic and the syntax that we learned above, the query will look like:
Output:
As you can see, the The For the To study more about the accumulator operators, please visit the MongoDB official documentation. Let’s take a look at another use case. Let’s say we are
interested in the number of people having a similar hobby. To achieve this, we should make every hobby a separate top-level field by using the The This allows more flexibility to query the arrays and data in the database. Then we have to do the same as we did in the first
example because, hobbies is not an array but a top-level field in a key-value form, and applying the
Output:
To study more about the Applications of $group operator:
I hope you enjoyed reading this article as much as I enjoyed writing it. Happy Coding! Peer Review Contributions by: Mohan Raj What is difference between $Group and $project in MongoDB?$group is used to group input documents by the specified _id expression and for each distinct grouping, outputs a document. $project is used to pass along the documents with the requested fields to the next stage in the pipeline.
What field is mandatory in a $group operation MongoDB?_id: This field is mandatory for grouping. If you specify the value of the _id field as null or a constant, the $group operator counts the accumulated values for all input documents as a whole.
How does MongoDB aggregate work?What is Aggregation in MongoDB? Aggregation is a way of processing a large number of documents in a collection by means of passing them through different stages. The stages make up what is known as a pipeline. The stages in a pipeline can filter, sort, group, reshape and modify documents that pass through the pipeline.
How does MongoDB lookup work?The MongoDB Lookup operator, by definition, “Performs a left outer join to an unshared collection in the same database to filter in documents from the “joined” collection for processing.” Simply put, using the MongoDB Lookup operator makes it possible to merge data from the document you are running a query on and the ...
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