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This text is a practical guide to performing basic operations in MongoDB, one of the most popular NoSQL databases for storing and managing data. It focuses on essential topics like CRUD operations (Create, Read, Update, Delete), customizing queries with sorting, pagination, and projections, and more advanced techniques such as the Aggregation Framework.

In addition to these, this guide touches on:

Using query operators for filtering data (e.g., $eq, $gt, $in), working with nested fields, utilizing logical operators ($and, $or, $not), managing arrays with operators like $push and $unwind, and even performing joins between collections with $lookup. Whether you’re a beginner looking to get started or an experienced user aiming to brush up on key concepts, this guide provides a clear and concise overview of MongoDB's most practical and widely used features.

1. CRUD Operations (Create, Read, Update, Delete)

1.1 Create - Inserting Data

insertOne(): Insert a single document.

db.collection.insertOne({ name: "John", age: 25 });

insertMany(): Insert multiple documents.

db.collection.insertMany([{ name: "Maria" }, { name: "Peter" }]);

1.2 Read - Querying Data

find(): Retrieve multiple documents.

db.collection.find({ age: { $gte: 18 } });

findOne(): Retrieve a single document.

db.collection.findOne({ name: "John" });

Query Operators

$eq: Equal to.
$ne: Not equal to.
$gt, $lt, $gte, $lte: Numeric comparisons.
$in, $nin: Check if a value is inside/outside a list.
$exists: Check if a field exists.

1.3 Update - Updating Data

updateOne(): Update a single document.

db.collection.updateOne({ name: "John" }, { $set: { age: 30 } });

updateMany(): Update multiple documents.

db.collection.updateMany({ age: { $lt: 18 } }, { $inc: { age: 1 } });

Update Operators

$set: Modify fields.
$inc: Increment numeric values.
$push, $pull: Add/remove elements from arrays.

1.4 Delete - Deleting Data

deleteOne(): Delete a single document.

db.collection.deleteOne({ name: "Peter" });

deleteMany(): Delete multiple documents.

db.collection.deleteMany({ age: { $lt: 18 } });

2. Customizing Queries

2.1 Field Projection

Return specific fields:

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

2.2 Sorting and Pagination

Sorting:
```
db.collection.find().sort({ age: 1 });
```

Pagination:

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

2.3 Regular Expressions

Partial search:
```
db.collection.find({ name: /john/i });
```

2.4 Querying Nested Fields

Query within subfields:

db.collection.find({ "address.city": "New York" });

2.5 Logical Operators

$and:

db.collection.find({ $and: [{ age: { $gte: 18 } }, { city: "Los Angeles" }] });

$or:

db.collection.find({ $or: [{ city: "New York" }, { city: "San Francisco" }] });

3. Aggregation Framework

3.1 Aggregation Pipeline

A sequence of stages to transform data:

db.collection.aggregate([
  { $match: { age: { $gte: 18 } } },
  { $group: { _id: "$city", total: { $sum: 1 } } }
]);

3.2 Key Stages

3.2.1 $match (Filtering Data)

Filter documents in the pipeline:
```
{ $match: { age: { $gte: 18 } } }
```

3.2.2 $project (Data Transformation)

Create or modify fields:

{ $project: { fullName: { $concat: ["$firstName", " ", "$lastName"] } } }

3.2.3 $group (Data Grouping)

Group by a field and perform calculations:

{ $group: { _id: "$city", total: { $sum: 1 } } }

3.2.4 Sorting and Limiting

Sorting:
```
{ $sort: { total: -1 } }
```
Limiting:
```
{ $limit: 10 }
```

3.2.5 Array Manipulation

$unwind: Expand arrays into individual documents:
```
{ $unwind: "$hobbies" }
```

3.2.6 $lookup (Relationships)

Perform "joins" between collections:

{
  $lookup: {
    from: "orders",
    localField: "userId",
    foreignField: "userId",
    as: "userOrders"
  }
}

3.2.7 $out (Export Results)

Export results to a new collection:
```
{ $out: "newCollection" }
```

MongoDB - A Practical Guide for Beginners and Experts Alike