Querying in NoSQL

NoSQL (Not Only SQL) is a non-relational database that stores data in flexible formats instead of traditional tables. It is designed to handle large volumes of structured, semi-structured and unstructured data efficiently.

• Data is commonly stored as documents, key-value pairs, columns or graphs.
• Documents are typically stored in JSON-like formats.

NoSQL Query Syntax

NoSQL databases use filters, logical operators and conditional operators to retrieve documents that match specific criteria. A query consists of a collection name and a set of conditions that determine which documents should be returned.

Basic Query Structure

The basic structure of a NoSQL query is:

db.collection.find({ condition })

Here:

• db represents the database.
• collection represents the collection containing the documents.
• find() is used to retrieve documents.
• condition specifies the filtering criteria.

Filtering Operators

Filtering operators are used to apply conditions and retrieve matching documents. The commonly used filtering operators are listed below.

Field Match

Field matching is used to retrieve documents where a field contains a specific value.

db.users.find({ name: "John" })

Multiple Field Match

Multiple fields can be matched by specifying more than one field-value pair in a query.

db.users.find({
  name: "John",
  city: "Delhi"
})

This query is equivalent to:

db.users.find({
  $and: [
    { name: "John" },
    { city: "Delhi" }
  ]
})

Match Sub Fields

Sub-fields in embedded documents can be queried using dot notation.

db.users.find({
  "address.city": "Delhi"
})

$regex Operator

The $regex operator matches field values against a regular expression pattern.

db.users.find({
  name: { $regex: "^Jo" }
})

$startsWith Operator

The $startsWith operator returns documents whose field values begin with a specified string.

db.users.find({
  name: { $startsWith: "Jo" }
})

$endsWith Operator

The $endsWith operator returns documents whose field values end with a specified string.

db.users.find({
  name: { $endsWith: "hn" }
})

Logical Operators

Logical operators are used to combine multiple query conditions in a NoSQL query.The following table summarizes the commonly used logical operators and their corresponding query syntax.

$and Operator

The $and operator returns documents that satisfy all specified conditions.

db.users.find({
  $and: [
    { city: "Delhi" },
    { age: 25 }
  ]
})

$or Operator

The $or operator returns documents that satisfy at least one of the specified conditions.

db.users.find({
  $or: [
    { city: "Delhi" },
    { city: "Mumbai" }
  ]
})

$not Operator

The $not operator returns documents that do not match a specified condition.

db.users.find({
  status: { $not: "Inactive" }
})

$in Operator

The $in operator returns documents whose field value matches any value in the specified array.

db.users.find({
  city: {
    $in: ["Delhi", "Mumbai"]
  }
})

$nin Operator

The $nin operator returns documents whose field value does not match any value in the specified array.

db.users.find({
  city: {
    $nin: ["Delhi", "Mumbai"]
  }
})

Conditional Operators

Conditional operators are used to compare field values and filter documents based on specific conditions. The following table summarizes the commonly used conditional operators and their query syntax.

$gt Operator

The $gt operator returns documents whose field value is greater than the specified value.

db.users.find({
  salary: { $gt: 50000 }
})

$gte Operator

The $gte operator returns documents whose field value is greater than or equal to the specified value.

db.users.find({
  salary: { $gte: 50000 }
})

$lt Operator

The $lt operator returns documents whose field value is less than the specified value.

db.users.find({
  salary: { $lt: 50000 }
})

$lte Operator

The $lte operator returns documents whose field value is less than or equal to the specified value.

db.users.find({
  salary: { $lte: 50000 }
})

$ne Operator

The $ne operator returns documents whose field value is not equal to the specified value.

db.users.find({
  status: { $ne: "Inactive" }
})

$exists Operator

The $exists operator checks whether a field exists in a document.

db.users.find({
  email: { $exists: true }
})

$elemMatch Operator

The $elemMatch operator matches documents containing at least one array element that satisfies the specified condition.

db.users.find({
  skills: {
    $elemMatch: {
      name: "Python"
    }
  }
})

$date Operator

The $date operator is used to filter documents based on date values.

db.users.find({
  createdAt: {
    $gt: {
      $date: "2024-01-01"
    }
  }
})