Cheatsheet - Cheatsheet

Shell & Database Commands

Command	Description
mongosh	Starts the MongoDB Shell and connects to a local instance.
mongosh "mongodb://host:port"	Connects the shell to a specific host and port.
show dbs	Lists all databases on the server.
use <db_name>	Switches to a database. If it doesn't exist, it's created upon first data insertion.
db	Shows the current database.
db.stats()	Displays stats for the current database.
db.dropDatabase()	Deletes the entire current database.
exit or Ctrl+D	Exits the MongoDB shell.

Collection Commands

Collections are analogous to tables in SQL databases.

Command	Description
show collections	Lists all collections in the current database.
db.createCollection("coll_name")	Explicitly creates a new collection.
db.coll_name.stats()	Displays stats for the specified collection.
db.coll_name.drop()	Deletes the entire collection.
db.coll_name.renameCollection("new_name")	Renames a collection.

CRUD Operations: Create (Insert)

• Insert a Single Document:

  db.inventory.insertOne({
    item: 'canvas',
    qty: 100,
    tags: ['cotton'],
    size: { h: 28, w: 35.5, uom: 'cm' },
  });
  

• Insert Multiple Documents:

  db.inventory.insertMany([
    {
      item: 'journal',
      qty: 25,
      tags: ['blank', 'red'],
      size: { h: 14, w: 21, uom: 'cm' },
    },
    {
      item: 'mat',
      qty: 85,
      tags: ['gray'],
      size: { h: 27.9, w: 35.5, uom: 'cm' },
    },
  ]);
  

CRUD Operations: Read (Query)

The find() method is used for all queries.

• Find All Documents:

  db.inventory.find({});
  

• Find a Single Document:

  db.inventory.findOne({ item: 'journal' });
  

• Pretty Print Results:

  db.inventory.find({}).pretty();
  

Query Filters (The first argument to find())

• Equality:

  // Find documents where qty is 25
  db.inventory.find({ qty: 25 });
  
  // Find documents with a nested field match
  db.inventory.find({ 'size.h': 14 });
  

• Comparison Operators:

  Operator	Meaning	Example
  $eq	Equals	{ qty: { $eq: 25 } } (same as { qty: 25 })
  $gt	Greater Than	{ qty: { $gt: 20 } }
  $gte	Greater Than or Equal	{ qty: { $gte: 25 } }
  $lt	Less Than	{ qty: { $lt: 30 } }
  $lte	Less Than or Equal	{ qty: { $lte: 25 } }
  $ne	Not Equal	{ item: { $ne: "canvas" } }
  $in	In an Array of values	{ qty: { $in: [25, 85] } }
  $nin	Not in an Array of values	{ item: { $nin: ["journal", "mat"] } }

• Logical Operators:

  // AND (implicit when using comma-separated fields)
  db.inventory.find({ item: 'mat', qty: { $gt: 50 } });
  
  // OR
  db.inventory.find({ $or: [{ item: 'journal' }, { qty: { $lt: 50 } }] });
  

• Array Operators:

  // Find where the tags array contains "red"
  db.inventory.find({ tags: 'red' });
  
  // Find where the tags array contains both "gray" and "canvas"
  db.inventory.find({ tags: { $all: ['gray', 'canvas'] } });
  

Cursors (Modifying Query Results)

Chain these methods after a find() call.

• Projections (Selecting specific fields):

  // Return only the item and qty fields (and _id by default)
  db.inventory.find({}, { item: 1, qty: 1 });
  
  // Exclude the _id field
  db.inventory.find({}, { item: 1, qty: 1, _id: 0 });
  

• Sorting: 1 for ascending, -1 for descending.

  db.inventory.find({}).sort({ qty: -1 });
  

• Limiting & Skipping (for pagination):

  // Get the first 5 documents
  db.inventory.find({}).limit(5);
  
  // Skip the first 5 documents and get the next 5
  db.inventory.find({}).skip(5).limit(5);
  

• Counting:

  db.inventory.countDocuments({ item: 'journal' });
  

CRUD Operations: Update

• Update a Single Document:

  db.inventory.updateOne(
    { item: 'journal' }, // Filter
    { $set: { 'size.uom': 'in' }, $currentDate: { lastModified: true } } // Update operator
  );
  

• Update Multiple Documents:

  db.inventory.updateMany(
    { qty: { $lt: 50 } }, // Filter
    { $set: { status: 'low_stock' } } // Update operator
  );
  

• Replace a Single Document:

  db.inventory.replaceOne(
    { item: 'canvas' }, // Filter
    { item: 'canvas_new', qty: 120, tags: ['paint'] } // Replacement document
  );
  

Common Update Operators

Operator	Description
$set	Sets the value of a field. Creates the field if it doesn't exist.
$unset	Removes the specified field from a document.
$inc	Increments the value of a field by a specified amount.
$rename	Renames a field.
$currentDate	Sets the value of a field to the current date.
$push	Appends a value to an array.
$pull	Removes all array elements that match a specified query.
$addToSet	Adds elements to an array only if they do not already exist.

CRUD Operations: Delete

• Delete a Single Document:

  db.inventory.deleteOne({ item: 'canvas_new' });
  

• Delete Multiple Documents:

  db.inventory.deleteMany({ status: 'low_stock' });
  

  Warning: db.inventory.deleteMany({}) will delete all documents in the collection.

Indexes

Indexes improve query performance.

Command	Description
db.collection.createIndex({ field: 1 })	Creates an ascending index on field. Use -1 for descending.
db.collection.createIndex({ field1: 1, field2: -1 })	Creates a compound index.
db.collection.createIndex({ field: 1 }, { unique: true })	Creates a unique index.
db.collection.getIndexes()	Lists all indexes on a collection.
db.collection.dropIndex("index_name")	Drops a specific index.

Aggregation Framework

Performs multi-stage data processing.

• Basic Pipeline Structure:

  db.collection.aggregate([
    { $stage1: { ... } },
    { $stage2: { ... } },
    // ...
  ])
  

• Example: Group by a field and calculate an average.

  db.orders.aggregate([
    // Stage 1: Filter for orders with status "A"
    { $match: { status: 'A' } },
  
    // Stage 2: Group by customer ID and sum the amounts
    { $group: { _id: '$cust_id', total: { $sum: '$amount' } } },
  
    // Stage 3: Sort by the new total field
    { $sort: { total: -1 } },
  ]);