Query & Projection Operator - ( Querying and Projection )

MongoDB provides powerful query and projection operators to help you retrieve and transform data in your collections. In this page, we'll discuss how to use query and projection operators in MongoDB.

Syntax

Query operators in MongoDB are used to filter and retrieve specific documents from a collection. Projection operators are used to transform or modify the documents returned by a query. Here's the syntax for using query and projection operators:

db.collection.find( { field: { operator: value } }, { field: { operator: value } } )

Where:

• db.collection.find is the function used to retrieve documents from a collection.
• { field: { operator: value } } is the query criteria used to filter the documents in the collection.
• { field: { operator: value } } is the projection criteria used to transform the documents returned by the query.

Example

Let's consider a "users" collection in MongoDB that has documents with the following structure:

{
    "id": 1,
    "name": "John Doe",
    "age": 30,
    "email": "johndoe@example.com"
}

Querying

To retrieve all documents where the "age" field is greater than 25, we can use the $gt (greater than) query operator as follows:

db.users.find( { age: { $gt: 25 } } )

This will return all documents where the "age" field is greater than 25.

Projection

To retrieve only the "name" and "email" fields for all documents where the "age" field is greater than 25, we can use the $project projection operator as follows:

db.users.find( { age: { $gt: 25 } }, { name: 1, email: 1, _id: 0 } )

This will return only the "name" and "email" fields for all documents where the "age" field is greater than 25, while excluding the "_id" field from the result set.

Output

When you execute a query and projection operation, MongoDB returns a cursor object that you can use to iterate over the result set and retrieve individual documents.

Explanation

Query operators in MongoDB allow you to filter documents based on specific criteria. Projection operators allow you to transform or modify the documents returned by a query. These operators are used in combination to refine the result set returned by a query.

Use

Query and projection operators are used to retrieve documents from MongoDB collections based on specific criteria and to transform or modify the documents returned by a query. They can be used to build complex database queries that retrieve only the data you need.

Important Points

• MongoDB has a large number of query and projection operators available that can be used to filter and transform documents.
• Query and projection operators can be used in combination to create complex queries that return only the data you need.
• The result set of a query can be iterated using a cursor object returned by MongoDB.

Summary

In this page, we discussed how to use query and projection operators in MongoDB. We covered the syntax, example, output, explanation, use, important points, and summary of query and projection operators. By using query and projection operators, you can build powerful queries and transformations to retrieve and manipulate data in MongoDB collections.