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Databases and Collections

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MongoDB stores BSON documents, i.e. data records, in collections; the collections in databases.

Databases

In MongoDB, databases hold collections of documents.

To select a database to use, in the mongo shell, issue the use <db> statement, as in the following example:

use myDB

Create a Database

If a database does not exist, MongoDB creates the database when you first store data for that database. As such, you can switch to a non-existent database and perform the following operation in the mongo shell:

use myNewDB

db.myNewCollection1.insert( { x: 1 } )

The insert() operation creates both the database myNewDB and the collection myNewCollection1 if they do not already exist.

For a list of restrictions on database names, see Naming Restrictions.

Collections

MongoDB stores documents in collections. Collections are analogous to tables in relational databases.

Create a Collection

If a collection does not exist, MongoDB creates the collection when you first store data for that collection.

db.myNewCollection2.insert( { x: 1 } )
db.myNewCollection3.createIndex( { y: 1 } )

Both the insert() and the createIndex() operations create their respective collection if they do not already exist.

For a list of restrictions on database names, see Naming Restrictions.

Explicit Creation

MongoDB provides the db.createCollection() method to explicitly create a collection with various options, such as setting the maximum size. If you are not specifying these options, you do not need to explicitly create the collection since MongoDB creates new collections when you first store data for the collections.

To modify these collection options, see collMod.

Modifying Document Structure

To change the structure of the documents in a collection, such as add new fields, remove existing fields, or change the field values to a new type, update the documents to the new structure.