Create databases and collections

The use command selects a database; it does not persist an empty database by itself. Inserting data or explicitly creating a collection establishes stored data structures.

Before you can store documents in MongoDB, you need a database and a collection. Think of a database as a folder, and a collection as a file inside that folder that holds your JSON-like documents.

To start the Mongo shell, run:

mongo

Now, to create or switch to a database:

use myFirstDB

switched to db myFirstDB

Note: The database is not created immediately. It only gets created when you add a collection and insert data into it.

Creating a Collection

There are two ways to create a collection:

• Implicitly by inserting a document into it
• Explicitly using db.createCollection()

Method 1: Implicit Creation (Most Common)

db.users.insertOne({
  name: "Alice",
  age: 25
});

{
  acknowledged: true,
  insertedId: ObjectId("...")
}

This command automatically creates the users collection and inserts the document.

db.createCollection("products")

{ ok: 1 }

You can now insert documents into the products collection.

Inserting into the Explicitly Created Collection

db.products.insertOne({
  name: "Laptop",
  price: 75000,
  inStock: true
});

{
  acknowledged: true,
  insertedId: ObjectId("...")
}

Intuition Check

Q: Why doesn't MongoDB create the database immediately when you type use myFirstDB?

A: Because MongoDB uses lazy creation. A database is only created when data is actually inserted. This saves space and keeps the system efficient.

Listing All Databases

show dbs

admin     0.000GB
config    0.000GB
local     0.000GB
myFirstDB 0.000GB  (only appears if a document was inserted)

show collections

products
users

Intuition Check

Q: Can you insert data into a collection before creating the database?

A: Yes! MongoDB automatically creates the database and the collection for you as soon as you insert data.

The use command selects a database; it does not persist an empty database by itself. Inserting data or explicitly creating a collection establishes stored data structures.

Summary

• use dbName switches or creates a database
• db.collection.insertOne() creates the collection (and database) implicitly
• db.createCollection("name") explicitly creates a collection
• show dbs and show collections help you view the structure

In the next lesson, we will learn how to insert multiple documents and understand how data is stored in MongoDB collections.

• Insert documents into a collection