Last modified: September 29, 2019

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Querying NoSQL Databases

Querying NoSQL databases requires a different approach compared to relational databases due to their diverse data models and storage mechanisms. This guide focuses on MongoDB, a popular NoSQL database, and explores how to query data effectively using its powerful query language.

After reading the material, you should be able to answer the following questions:

1. How does MongoDB's document model differ from traditional relational databases, and what advantages does it offer for schema design?
2. What are the key differences between basic queries (like find, countDocuments, and distinct) and advanced queries (such as aggregation, text search, and geospatial queries) in MongoDB?
3. How do indexing strategies in MongoDB, including single field, compound, multikey, text, and geospatial indexes, enhance query performance?
4. What is the purpose of the explain() method in MongoDB, and how can it be used to analyze and optimize query execution plans?
5. In what scenarios would you choose to use specific types of indexes (e.g., text indexes for search functionality or geospatial indexes for location-based queries) in MongoDB?

Introduction to MongoDB

Data Model

• MongoDB stores data in documents, which are JSON-like objects capable of containing nested documents and arrays.
• Each document is stored within a collection, analogous to a table in relational databases.
• The document model allows for complex data structures and offers flexibility in schema design.

Example Document:

{
  "_id": ObjectId("507f1f77bcf86cd799439011"),
  "name": "John Doe",
  "age": 30,
  "email": "john.doe@example.com",
  "addresses": [
    {
      "street": "123 Main St",
      "city": "Anytown",
      "zip": "12345"
    },
    {
      "street": "456 Elm St",
      "city": "Othertown",
      "zip": "67890"
    }
  ]
}

Query Language

• MongoDB uses a JSON-like syntax for queries, making it intuitive for developers familiar with JSON.
• The query language provides a rich set of operators for filtering, projection, sorting, and aggregation.

Examples of Query Operators:

• $eq, $ne: Equal, Not Equal
• $gt, $gte, $lt, $lte: Greater Than, Greater Than or Equal, Less Than, Less Than or Equal
• $in, $nin: In, Not In
• $and, $or, $not, $nor: Logical Operators

Basic Queries

Find

• The find method retrieves documents from a collection that match specified filter criteria.
• The syntax for find is db.collection.find(query, projection), where query specifies selection criteria and projection determines the fields to include or exclude.

Example: Retrieve all users aged 25

Sample Data in users Collection:

Suppose we have the following documents in our users collection:

db.users.insertMany([
  { _id: 1, name: "Alice", age: 25, email: "alice@example.com", city: "New York" },
  { _id: 2, name: "Bob", age: 30, email: "bob@example.com", city: "Los Angeles" },
  { _id: 3, name: "Carol", age: 25, email: "carol@example.com", city: "Chicago" },
  { _id: 4, name: "Dave", age: 28, email: "dave@example.com", city: "New York" }
])

Code:

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

Result:

{ "_id" : 1, "name" : "Alice", "age" : 25, "email" : "alice@example.com", "city" : "New York" }
{ "_id" : 3, "name" : "Carol", "age" : 25, "email" : "carol@example.com", "city" : "Chicago" }

Example with Projection: Retrieve users aged 25, but only show their names and emails

Code:

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

Result:

{ "name" : "Alice", "email" : "alice@example.com" }
{ "name" : "Carol", "email" : "carol@example.com" }

Count

• The count method returns the number of documents that match a query.
• The syntax for count is db.collection.countDocuments(query).

Example: Count the number of users aged 25

Code:

db.users.countDocuments({ age: 25 })

Result:

2

Distinct

• The distinct method finds the unique values for a specified field across a collection.
• The syntax for distinct is db.collection.distinct(field, query).

Example: Get a list of unique cities where users aged 25 live

Code:

db.users.distinct("city", { age: 25 })

Result:

[ "New York", "Chicago" ]

Advanced Queries

Aggregation

• Aggregation operations process data records and return computed results.
• MongoDB's aggregation framework provides an efficient way to perform data analysis using a pipeline of stages.

Syntax:

db.collection.aggregate(pipeline, options)

• The pipeline is an array of stages that process and transform the data.

Example: Group users by city and count the number of users in each city

Sample Data:

Using the same users collection as before.

Code:

db.users.aggregate([
  { $group: { _id: "$city", count: { $sum: 1 } } }
])

Result:

{ "_id" : "New York", "count" : 2 }
{ "_id" : "Los Angeles", "count" : 1 }
{ "_id" : "Chicago", "count" : 1 }

• The $group stage groups documents by the specified _id expression.
• The $sum: 1 increments the count by 1 for each document in the group.

Example: Calculate the average age of users in each city

Code:

db.users.aggregate([
  { $group: { _id: "$city", averageAge: { $avg: "$age" } } }
])

Result:

{ "_id" : "New York", "averageAge" : 26.5 }
{ "_id" : "Los Angeles", "averageAge" : 30 }
{ "_id" : "Chicago", "averageAge" : 25 }

Text Search

MongoDB supports text search through text indexes, allowing you to perform search operations on string content.

Sample Data in articles Collection:

db.articles.insertMany([
  { _id: 1, title: "Introduction to MongoDB", content: "MongoDB is a NoSQL database." },
  { _id: 2, title: "NoSQL Databases", content: "NoSQL databases are non-relational." },
  { _id: 3, title: "Relational Databases", content: "SQL databases are relational." },
  { _id: 4, title: "Advantages of NoSQL", content: "NoSQL databases like MongoDB are scalable." }
])

Creating a Text Index:

db.articles.createIndex({ content: "text" })

The syntax for text search is db.collection.find({ $text: { $search: searchString } }).

Example: Find articles that contain the word "NoSQL"

Code:

db.articles.find({ $text: { $search: "NoSQL" } })

Result:

{ "_id" : 1, "title" : "Introduction to MongoDB", "content" : "MongoDB is a NoSQL database." }
{ "_id" : 2, "title" : "NoSQL Databases", "content" : "NoSQL databases are non-relational." }
{ "_id" : 4, "title" : "Advantages of NoSQL", "content" : "NoSQL databases like MongoDB are scalable." }

Advanced Text Search:

For phrase search, enclose the phrase in double quotes.

Example: Find articles containing the phrase "NoSQL databases"

Code:

db.articles.find({ $text: { $search: "\"NoSQL databases\"" } })

Result:

{ "_id" : 2, "title" : "NoSQL Databases", "content" : "NoSQL databases are non-relational." }
{ "_id" : 4, "title" : "Advantages of NoSQL", "content" : "NoSQL databases like MongoDB are scalable." }

To exclude terms, use a minus sign before the word.

Example: Find articles that contain "NoSQL" but not "MongoDB"

Code:

db.articles.find({ $text: { $search: "NoSQL -MongoDB" } })

Result:

{ "_id" : 2, "title" : "NoSQL Databases", "content" : "NoSQL databases are non-relational." }

Geospatial Queries

• MongoDB provides powerful geospatial indexing and querying capabilities for location-based data.

Storing Location Data:

Sample Data in places Collection:

db.places.insertMany([
  {
    _id: 1,
    name: "Central Park",
    location: {
      type: "Point",
      coordinates: [-73.9667, 40.78]
    }
  },
  {
    _id: 2,
    name: "Times Square",
    location: {
      type: "Point",
      coordinates: [-73.9855, 40.7580]
    }
  },
  {
    _id: 3,
    name: "Empire State Building",
    location: {
      type: "Point",
      coordinates: [-73.9857, 40.7484]
    }
  }
])

Creating a 2dsphere Index:

db.places.createIndex({ location: "2dsphere" })

The syntax for geospatial queries is:

db.collection.find({
  location: {
    $near: {
      $geometry: point,
      $maxDistance: distance
    }
  }
})

Example: Find places within 1,000 meters of Times Square

Code:

db.places.find({
  location: {
    $near: {
      $geometry: {
        type: "Point",
        coordinates: [-73.9855, 40.7580]
      },
      $maxDistance: 1000  // distance in meters
    }
  }
})

Result:

Assuming that the Empire State Building is within 1,000 meters of Times Square:

{ "_id" : 2, "name" : "Times Square", "location" : { "type" : "Point", "coordinates" : [ -73.9855, 40.758 ] } }
{ "_id" : 3, "name" : "Empire State Building", "location" : { "type" : "Point", "coordinates" : [ -73.9857, 40.7484 ] } }

• The $near operator finds documents near a specified point.
• The $geometry field defines the point with coordinates.
• The $maxDistance sets the maximum distance from the point in meters.

Indexing in MongoDB

Creating Indexes

• Indexes support efficient query execution by limiting the number of documents that MongoDB needs to examine.
• The syntax for creating an index is db.collection.createIndex(keys, options), where keys specifies the field or fields to index.

Example: Create an index on the age field

db.users.createIndex({ age: 1 })

Use 1 for ascending order and -1 for descending order.

Compound Indexes:

Indexes can be created on multiple fields, known as compound indexes.

db.users.createIndex({ age: 1, city: 1 })

Types of Indexes

• Single Field Indexes: Indexes on a single field improve query performance on that field.
• Compound Indexes: Indexes on multiple fields support queries that sort or filter on multiple fields.
• Multikey Indexes: Indexes on array fields enable efficient querying of documents with array data.
• Text Indexes: Indexes that enable text search functionality over string content.
• Geospatial Indexes: Indexes that support geospatial queries for location data.

Index Usage

Use the explain() method to understand how MongoDB executes a query and whether it utilizes an index.

db.users.find({ age: 25 }).explain("executionStats")

• Analyze the output to check for "stage": "IXSCAN" to confirm index usage.
• Check "nReturned" and "totalKeysExamined" for insights into query performance.

Monitoring Indexes:

Use db.collection.getIndexes() to list all indexes on a collection.

db.users.getIndexes()

Dropping Indexes:

Remove unnecessary indexes to optimize performance and reduce storage overhead.

db.users.dropIndex("age_1")