What is a MongoDB Query?

MongoDB Query is a fundamental aspect of MongoDB that allows users to fetch data from the database. Similar to SQL queries in traditional databases, MongoDB queries provide simplicity and flexibility in retrieving specific data based on certain criteria or conditions.

In this article, We will learn about the MongoDB Query in detail by performing various Queries on document of MongoDB collection and so on.

• MongoDB Query allows retrieving data from the MongoDB database. MongoDB Query provides simplicity in the process of fetching data from the database, it’s similar to SQL queries in SQL Database language.
• While performing a query operation, one can also use criteria or conditions that can be used to retrieve specific data from the database. 
• MongoDB provides the function names as db.collection_name.find() to operate query operations on the database. 

Set Up an Environment:

To understand MongoDB Query we need a collection called articles on which we will perform various operations and queries. Below are the sample documents which we will use in the article.

[
  {
    "_id": ObjectId("6009585d35cce6b7b8f087f1"),
    "title": "Math",
    "author": "Aditya",
    "level": "basic",
    "length": 230,
    "example": 11
  },
  {
    "_id": ObjectId("60095b8a3fc110f90873ce29"),
    "title": "Array",
    "author": "Aditya",
    "level": "basic",
    "length": 200,
    "example": 5
  },
  {
    "_id": ObjectId("60095b8a3fc110f90873ce2a"),
    "title": "Stack",
    "author": "Rakesh",
    "level": "easy",
    "length": 400,
    "example": 10
  },
  {
    "_id": ObjectId("60095b8a3fc110f90873ce2b"),
    "title": "Queue",
    "author": "Rakesh",
    "level": "medium",
    "length": 350,
    "example": 2
  },
  {
    "_id": ObjectId("60095b8a3fc110f90873ce2c"),
    "title": "Tree",
    "author": "devil",
    "level": "high",
    "length": 1000,
    "example": 10
  },
  {
    "_id": ObjectId("60095b8a3fc110f90873ce2d"),
    "title": "Graph",
    "author": "Aditya",
    "level": "high",
    "length": 1500,
    "example": 15
  },
  {
    "_id": ObjectId("60095b8d3fc110f90873ce2e"),
    "title": "Segment Tree",
    "author": "devil",
    "level": "very high",
    "length": 500,
    "example": 20
  },
  {
    "_id": ObjectId("60095fd4e5a7731b2a55a922"),
    "title": "Trie",
    "author": "Rakesh",
    "length": 500,
    "example": 20,
    "time": 50
  }
]

In MongoDB, we can find a single document using findOne() method, This method returns the first document that matches the given filter query expression.

Syntax:  

db.collection_name.findOne ()

Example:

db.article.findOne()

Output:

test> db.article.findOne()
{
  _id: ObjectId('6009585d35cce6b7b8f087f1'),
  title: 'Math',
  author: 'Aditya',
  level: 'basic',
  length: 230,
  example: 11
}

Here, we are going to display the first document of the article collection.

Example:

Retrieve and display all documents from the article.

db.article.find().pretty() 

Output:

[
  {
    _id: ObjectId('6009585d35cce6b7b8f087f1'),
    title: 'Math',
    author: 'Aditya',
    level: 'basic',
    length: 230,
    example: 11
  },
  {
    _id: ObjectId('60095b8a3fc110f90873ce29'),
    title: 'Array',
    author: 'Aditya',
    level: 'basic',
    length: 200,
    example: 5
  },
  {
    _id: ObjectId('60095b8a3fc110f90873ce2a'),
    title: 'Stack',
    author: 'Rakesh',
    level: 'easy',
    length: 400,
    example: 10
  },
  {
    _id: ObjectId('60095b8a3fc110f90873ce2b'),
    title: 'Queue',
    author: 'Rakesh',
    level: 'medium',
    length: 350,
    example: 2
  },
  {
    _id: ObjectId('60095b8a3fc110f90873ce2c'),
    title: 'Tree',
    author: 'devil',
    level: 'high',
    length: 1000,
    example: 10
  },
  {
    _id: ObjectId('60095b8a3fc110f90873ce2d'),
    title: 'Graph',
    author: 'Aditya',
    level: 'high',
    length: 1500,
    example: 15
  },
  {
    _id: ObjectId('60095b8d3fc110f90873ce2e'),
    title: 'Segment Tree',
    author: 'devil',
    level: 'very high',
    length: 500,
    example: 20
  },
  {
    _id: ObjectId('60095fd4e5a7731b2a55a922'),
    title: 'Trie',
    author: 'Rakesh',
    length: 500,
    example: 20,
    time: 50
  }
]

In the above Query, We have display the documents of the article collection in a well-formatted way using pretty() method.

The equality operator($eq) is used to match the documents where the value of the field is equal to the specified value. In other words, the $eq operator is used to specify the equality condition.

Example: 

Retrieve and display all documents from the ‘article’ collection where the author is “devil”.

db.article.find({author:{$eq:"devil"}}).pretty()  

Output:

[
  {
    _id: ObjectId('60095b8a3fc110f90873ce2c'),
    title: 'Tree',
    author: 'devil',
    level: 'high',
    length: 1000,
    example: 10
  },
  {
    _id: ObjectId('60095b8d3fc110f90873ce2e'),
    title: 'Segment Tree',
    author: 'devil',
    level: 'very high',
    length: 500,
    example: 20
  }
]

In the above Query, We have display the documents that matches the filter query(i.e., {author  :  {$eq : “devil”}}) from the article collection.

To get the specific numeric data using conditions like greater than equal or less than equal use the $gte or $lte operator in the find() method. 

Syntax:  

db.collection_name.find({< key > : {$gte : < value >}}) 
or 
db.collection_name.find({< key > : {$lte : < value >}}) 

Example: 

Retrieve and display all documents from the ‘article’ collection where the length is greater than or equal to 510.

db.article.find({length:{$gte:510}}).pretty()

Output:

[
  {
    _id: ObjectId('60095b8a3fc110f90873ce2c'),
    title: 'Tree',
    author: 'devil',
    level: 'high',
    length: 1000,
    example: 10
  },
  {
    _id: ObjectId('60095b8a3fc110f90873ce2d'),
    title: 'Graph',
    author: 'Aditya',
    level: 'high',
    length: 1500,
    example: 15
  }
]

In the above Query, We have documented data which has the length attribute value greater than 510. So, we pass a filter query that is {length : {$gte : 510}} in the find() method.

$exists operator shows all the collection documents if they exist on a given key.

Example:  

db.article.find({time:{$exists:"true"}}).pretty()

Output:

[
  {
    _id: ObjectId('60095fd4e5a7731b2a55a922'),
    title: 'Trie',
    author: 'Rakesh',
    length: 500,
    example: 20,
    time: 50
  }
]

In the above Query, We have documents which has the attribute named as time by passing a filter query that is {time : {$exists : “true”}} in the find() method.

$and operator comes under the type of MongoDB logical operator which perform logical AND operation on the array of one or more expressions and select or retrieve only those documents that match all the given expression in the array.

Example:

db.article.find({$and:[{level:{$eq:"high"}},{level:{$exists : "true"}}]}).pretty()

Output:

[
  {
    _id: ObjectId('60095b8a3fc110f90873ce2c'),
    title: 'Tree',
    author: 'devil',
    level: 'high',
    length: 1000,
    example: 10
  },
  {
    _id: ObjectId('60095b8a3fc110f90873ce2d'),
    title: 'Graph',
    author: 'Aditya',
    level: 'high',
    length: 1500,
    example: 15
  }
]

In the above Query, We have using $and operator and given two condition which are highlighted following 

• and operator: {$and : [ first condition, second condition]}
• first condition(level == “high”): { level : {$eq  :  “high”}}
• second condition: {level : {$exists : “true”}} 

Example: 

Retrieve articles that are either authored by “Aditya” and have a level of “basic”, or authored by “Rakesh” with a level of “medium”.

db.article.find({
  $or: [
    { level: "basic", author: "Aditya" },
    { $and: [
        { level: "medium" },
        { author: "Rakesh" }
      ]
    }
  ]
}).pretty()

Output:

[
  {
    _id: ObjectId('6009585d35cce6b7b8f087f1'),
    title: 'Math',
    author: 'Aditya',
    level: 'basic',
    length: 230,
    example: 11
  },
  {
    _id: ObjectId('60095b8a3fc110f90873ce29'),
    title: 'Array',
    author: 'Aditya',
    level: 'basic',
    length: 200,
    example: 5
  },
  {
    _id: ObjectId('60095b8a3fc110f90873ce2b'),
    title: 'Queue',
    author: 'Rakesh',
    level: 'medium',
    length: 350,
    example: 2
  }
]

This query method specifies a maximum number of documents for a cursor to return. 

Example: 

db. article. find({author : "devil" }). limit(2) . pretty() 

Output : 

[
  {
    _id: ObjectId('60095b8a3fc110f90873ce2c'),
    title: 'Tree',
    author: 'devil',
    level: 'high',
    length: 1000,
    example: 10
  },
  {
    _id: ObjectId('60095b8d3fc110f90873ce2e'),
    title: 'Segment Tree',
    author: 'devil',
    level: 'very high',
    length: 500,
    example: 20
  }
]

This query method is simply the extension of the find method only provide a result at a maximum limited number(here is 2) 

• Query to find record having given attribute: find({author : “devil” }) 
• Query to limit result: limit( 2).

Overall, MongoDB queries are essential for interacting with MongoDB databases. By using the find() method and various query operators, users can retrieve specific documents that match their criteria. Whether it’s selecting single documents, filtering based on conditions, or combining multiple conditions using logical operators, MongoDB queries offer a powerful and efficient way to fetch data from MongoDB databases