# MongoDB

### MongoDB Basics

| Command             | Description                                                        |
| ------------------- | ------------------------------------------------------------------ |
| `mongod`            | Start MongoDB server                                               |
| `mongo`             | Open the MongoDB shell                                             |
| `show dbs`          | Show all the available databases                                   |
| `use db_name`       | Switch to the specified database or create one if it doesn't exist |
| `db`                | Display the current database you are using                         |
| `db.dropDatabase()` | Drop the current database                                          |

### CRUD Operations

#### Insert

| Command                                                 | Description                                   |
| ------------------------------------------------------- | --------------------------------------------- |
| `db.collection.insertOne(document)`                     | Insert a new document into the collection     |
| `db.collection.insertMany([document1, document2, ...])` | Insert multiple documents into the collection |

#### Find

| Command                        | Description                                            |
| ------------------------------ | ------------------------------------------------------ |
| `db.collection.find()`         | Retrieve all documents from the collection             |
| `db.collection.find(query)`    | Retrieve documents that match the specified query      |
| `db.collection.findOne()`      | Retrieve one document from the collection              |
| `db.collection.findOne(query)` | Retrieve one document that matches the specified query |

#### Update

| Command                                        | Description                                       |
| ---------------------------------------------- | ------------------------------------------------- |
| `db.collection.updateOne(query, update)`       | Update the first document that matches the query  |
| `db.collection.updateMany(query, update)`      | Update all documents that match the query         |
| `db.collection.replaceOne(query, replacement)` | Replace the first document that matches the query |

#### Delete

| Command                           | Description                                      |
| --------------------------------- | ------------------------------------------------ |
| `db.collection.deleteOne(query)`  | Delete the first document that matches the query |
| `db.collection.deleteMany(query)` | Delete all documents that match the query        |

### Query Operators

#### Comparison Operators

| Operator | Description                                                        | Example                                |
| -------- | ------------------------------------------------------------------ | -------------------------------------- |
| `$eq`    | Matches values that are equal to a specified value                 | `{ age: { $eq: 18 } }`                 |
| `$ne`    | Matches all values that are not equal to a specified value         | `{ age: { $ne: 18 } }`                 |
| `$gt`    | Matches values that are greater than a specified value             | `{ age: { $gt: 18 } }`                 |
| `$gte`   | Matches values that are greater than or equal to a specified value | `{ age: { $gte: 18 } }`                |
| `$lt`    | Matches values that are less than a specified value                | `{ age: { $lt: 18 } }`                 |
| `$lte`   | Matches values that are less than or equal to a specified value    | `{ age: { $lte: 18 } }`                |
| `$in`    | Matches any of the values specified in an array                    | `{ name: { $in: ['Alice', 'Bob'] } }`  |
| `$nin`   | Matches none of the values specified in an array                   | `{ name: { $nin: ['Alice', 'Bob'] } }` |

#### Logical Operators

| Operator | Description                              | Example                                            |
| -------- | ---------------------------------------- | -------------------------------------------------- |
| `$and`   | Joins query clauses with a logical AND   | `{ $and: [ { name: 'Alice' }, { age: 18 } ] }`     |
| `$or`    | Joins query clauses with a logical OR    | `{ $or: [ { name: 'Alice' }, { name: 'Bob' } ] }`  |
| `$not`   | Inverts the effect of a query expression | `{ age: { $not: { $gte: 18 } } }`                  |
| `$nor`   | Joins query clauses with a logical NOR   | `{ $nor: [ { name: 'Alice' }, { name: 'Bob' } ] }` |

#### Element Operators

| Operator  | Description                                     | Example                      |
| --------- | ----------------------------------------------- | ---------------------------- |
| `$exists` | Matches documents that have the specified field | `{ age: { $exists: true } }` |

#### Create collection in MongoDB

To create a collection in MongoDB, you can use the `db.createCollection()` method in the MongoDB shell. The syntax for creating a collection is as follows:

```
db.createCollection("<collection_name>", <options>)
```

Where `<collection_name>` is the name of the collection you want to create and `<options>` are the optional parameters that you can use to configure the collection. Here are some examples of how you can create a collection:

1. Create a collection without any options:

```
> db.createCollection("users")
```

This will create a collection named "users" in the current database.

2. Create a collection with options:

```
> db.createCollection("users", {
    capped: true,
    size: 100000,
    max: 100
  })
```

This will create a capped collection named "users" with a maximum size of 100,000 bytes and a maximum number of documents of 100.

You can also create a collection using the `MongoCollection` class in a MongoDB driver for your programming language. The specific syntax for doing this will depend on the driver you are using.