Basic Queries in MongoDB

Basic queries in MongoDB allow you to retrieve documents from a collection based on specified criteria. MongoDB’s query language is similar to JavaScript and provides a rich set of operators for filtering and retrieving data.

Query Syntax

MongoDB queries are structured as JSON-like documents. The basic syntax for querying documents is:

db.collection.find(query, projection)

• query: Specifies the criteria for selecting documents.
• projection: Specifies which fields to return (optional).

Basic Query Operations

1. Find All Documents
2. Find Documents by Criteria
3. Query Operators
4. Combining Conditions
5. Projections

1. Find All Documents

To retrieve all documents from a collection, use the find() method without any query criteria.

Syntax:

db.collection.find()

Example:

db.users.find()

This command returns all documents in the users collection.

2. Find Documents by Criteria

To find documents that match specific criteria, use a query document that specifies the search conditions.

Syntax:

db.collection.find(query)

Example:

Find users with the name "John Doe":

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

This command returns all documents in the users collection where the name field is "John Doe".

3. Query Operators

MongoDB supports a variety of query operators to specify more complex criteria:

• Comparison Operators:
  • $eq: Equal to
  • $ne: Not equal to
  • $gt: Greater than
  • $lt: Less than
  • $gte: Greater than or equal to
  • $lte: Less than or equal to

Syntax:

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

Example:

Find users older than 25:

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

Find products with a price less than or equal to 100:

db.products.find({ price: { $lte: 100 } })

• Logical Operators:
  • $and: Combines multiple conditions (implicit with multiple query fields)
  • $or: Matches documents that satisfy at least one condition
  • $not: Inverts the query
  • $nor: Matches documents that do not satisfy any of the conditions

Syntax:

db.collection.find({ $or: [ { condition1 }, { condition2 } ] })

Example:

Find users who are either named "John Doe" or are older than 30:

db.users.find({ $or: [ { name: "John Doe" }, { age: { $gt: 30 } } ] })

• Element Operators:
  • $exists: Checks for the existence of a field
  • $type: Checks the type of a field

Syntax:

db.collection.find({ field: { $exists: true } })

Example:

Find documents where the email field exists:

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

4. Combining Conditions

You can combine multiple query conditions to create more specific queries. MongoDB queries use implicit $and when specifying multiple conditions.

Syntax:

db.collection.find({ field1: value1, field2: value2 })

Example:

Find users who are named "John Doe" and are older than 25:

db.users.find({ name: "John Doe", age: { $gt: 25 } })

5. Projections

Projections control which fields are returned in the query results. By default, all fields are returned, but you can specify fields to include or exclude.

Syntax:

db.collection.find(query, projection)

• Include Fields:
  • { field: 1 }: Include the specified field(s)
• Exclude Fields:
  • { field: 0 }: Exclude the specified field(s)

Syntax:

db.collection.find(query, { field1: 1, field2: 1 })

Example:

Find all users but only return the name and age fields:

db.users.find({}, { name: 1, age: 1 })

Example:

Find all users but exclude the email field:

db.users.find({}, { email: 0 })

Example Queries

1. Find all users with a specific email:

   db.users.find({ email: "johndoe@example.com" })
   

2. Find products with a price between 50 and 150:

   db.products.find({ price: { $gte: 50, $lte: 150 } })
   

3. Find users who are not older than 25:

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

4. Find documents with a field that is not null:

   db.users.find({ email: { $ne: null } })