MongoDB Query API

The MongoDB Query API provides powerful methods to retrieve, filter, and shape documents from a database using flexible criteria and operators.
This guide covers essential query patterns and how to use operators, projections, sorting, and pagination for effective data retrieval.

1. Basic Queries

Basic queries search for documents based on direct key-value criteria within a collection.

Example:

db.collection.find({ key: value })

This command fetches all documents from collection where the field key matches value.

2. Comparison Operators

The Query API supports comparison operators to select documents matching conditions.

$eq: equal to (default, e.g., { age: 25 })
$ne: not equal to
$gt: greater than
$gte: greater than or equal to
$lt: less than
$lte: less than or equal to

Examples:

db.collection.find({ age: { $gt: 30 } })

db.collection.find({ price: { $lt: 100 } })

These queries fetch all documents where age is greater than 30 and price is less than 100, respectively.

3. Logical Operators

Logical operators combine multiple conditions in a single query, allowing for complex data retrieval.

$and: matches documents that satisfy all specified conditions
$or: matches documents that satisfy at least one specified condition
$nor: excludes documents that match any specified condition
$not: inverts condition

Examples:

db.collection.find({ $and: [{ key1: value1 }, { key2: value2 }] })

db.collection.find({ $or: [{ key1: value1 }, { key2: value2 }] })

The $and example returns documents satisfying both conditions. The $or query returns documents matching either.

4. Array Queries

MongoDB seamlessly queries fields containing arrays. You can match specific elements or find documents where arrays include at least one matching value.

Example:

db.collection.find({ tags: "mongodb" })

This returns documents where the tags array contains the string mongodb.

To match arrays with specific elements or with all given values, use operators like $all, $elemMatch, or $size for advanced array queries.

5. Projection

Projection lets you specify which fields to display in your query results: include (1) or exclude (0) fields as needed.

Example:

db.collection.find({}, { _id: 0, name: 1 })

This returns each document without the _id field, displaying only the name field. By default, _id is always returned unless specifically excluded.

6. Sorting Results

Use the sort() method to order query results by field.

1 — Ascending
-1 — Descending

Example:

db.collection.find().sort({ field: 1 })

This retrieves documents sorted in ascending order by field. For descending order, use field: -1.

7. Limit and Skip

paginating large result sets is simple with limit() and skip().

limit(n): restricts results to n documents.
skip(n): skips the first n documents.

Example:

db.collection.find().limit(10).skip(5)

This fetches 10 documents after skipping the first 5.

8. Additional Query Features

Use countDocuments() to get a count of matching documents.
Apply distinct() to retrieve unique values for a specific field.
Build compound queries using combinations of logical and comparison operators for flexible searching.