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CRUD: Read

Reading data is the most common database operation—and the one that can make or break your application’s performance. MongoDB provides a powerful query language that lets you find exactly the data you need, efficiently.

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Understanding MongoDB Queries

Unlike SQL’s string-based queries, MongoDB uses JSON-like query documents. This approach is:
  • Type-safe: No SQL injection vulnerabilities from string concatenation
  • Composable: Build complex queries programmatically
  • Intuitive: If you know JSON, you can write queries

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Query Performance Basics

Every query should be intentional. Poorly written queries can:
  • Scan millions of documents instead of using indexes
  • Consume excessive memory on large result sets
  • Block other operations due to collection-level locks (in older versions)
Always use projection to return only the fields you need, and consider indexes for frequently queried fields. We’ll cover indexing in a later chapter.

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Basic Query Methods

Read operations retrieve documents from a collection; i.e., query a collection for documents.

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find() and findOne()

  • find(filter): Returns a cursor to the matching documents.
  • findOne(filter): Returns the first matching document.
// Find all documents
db.users.find({})

// Find users with name "Alice"
db.users.find({ name: "Alice" })

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Comparison Operators

  • $eq: Equal to
  • $ne: Not equal to
  • $gt: Greater than
  • $gte: Greater than or equal to
  • $lt: Less than
  • $lte: Less than or equal to
  • $in: In an array
  • $nin: Not in an array
Examples: 
// Age > 25
db.users.find({ age: { $gt: 25 } })

// Status is "active" OR "pending"
db.users.find({ status: { $in: ["active", "pending"] } })

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Logical Operators

  • $or: Joins query clauses with a logical OR
  • $and: Joins query clauses with a logical AND
  • $not: Inverts the effect of a query expression
  • $nor: Joins query clauses with a logical NOR
Example: 
// Age < 30 AND status = "active"
db.users.find({
  $and: [
    { age: { $lt: 30 } },
    { status: "active" }
  ]
})

// Implicit AND (shorter syntax)
db.users.find({ age: { $lt: 30 }, status: "active" })

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Element Operators

  • $exists: Matches documents that have the specified field.
  • $type: Selects documents if a field is of the specified type.
// Find users who have a "phone" field
db.users.find({ phone: { $exists: true } })

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Projection

Projection determines which fields are returned in the matching documents. 
// Return only name and age (exclude _id)
db.users.find({}, { name: 1, age: 1, _id: 0 })

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Summary

  • Use find() to query data.
  • Use comparison operators like $gt, $lt, $in for filtering.
  • Use logical operators like $or, $and for complex logic.
  • Use Projection to limit the fields returned.