FAQ: MongoDB Fundamentals

What platforms does MongoDB support?

For the list of supported platforms, see Platform Support Notes.

Is MongoDB offered as a hosted service?

Yes. MongoDB Atlas is a cloud-hosted database-as-a-service. For more information, please visit MongoDB Atlas.

How does a collection differ from a table?

Instead of tables, a MongoDB database stores its data in collections. A collection holds one or more BSON documents. Documents are analogous to records or rows in a relational database table. Each document has one or more fields; fields are similar to the columns in a relational database table.

How do I create a database and a collection?

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

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

As such, you can switch to a non-existent database (use <dbname>) and perform the following operation:

use myNewDB;
db.myNewCollection1.insertOne( { x: 1 } );
db.myNewCollection2.createIndex( { a: 1 } );

• The db.collection.insertOne() method creates the collection myNewCollection1 if it does not already exist.

• The db.collection.createIndex() method creates the index and the collection myNewCollection2 if it does not already exist.

• If the myNewDb database did not exist, either the db.collection.createIndex() method or db.collection.insertOne() method would have created the myNewDb database automatically.

You can also create a collection explicitly using db.createCollection() method if you want to specify specific options, such as maximum size or document validation rules:

use myNewDB;
db.createCollection("myNewCollection1");

How do I define or alter the collection schema?

You do not need to specify a schema for a collection in MongoDB. Although it is common for the documents in a collection to have a largely homogeneous structure, it is not a requirement; i.e. documents in a single collection do not need to have the same set of fields. The data type for a field can differ across documents in a collection as well.

To change the structure of the documents in a collection, update the documents to the new structure. For instance, add new fields, remove existing ones, or update the value of a field to a new type.

Some collection properties, such as specifying a maximum size, can be specified during the explicit creation of a collection and be modified. See db.createCollection() and collMod. If you are not specifying these properties, you do not need to explicitly create the collection since MongoDB creates new collections when you first store data for the collections.

Does MongoDB support SQL?

Not directly, no. However, MongoDB does support a rich query language of its own. For examples on using MongoDB's query language, see MongoDB CRUD Operations

You can also use the MongoDB Connector for BI to query MongoDB collections with SQL.

If you are considering migrating your SQL application to MongoDB, download the MongoDB Application Modernization Guide for a best practices migration guide, reference schema, and other helpful resources.

Does MongoDB support transactions?

Because a single document can contain related data that would otherwise be modeled across separate parent-child tables in a relational schema, MongoDB's atomic single-document operations already provide transaction semantics that meet the data integrity needs of the majority of applications. One or more fields may be written in a single operation, including updates to multiple sub-documents and elements of an array. The guarantees provided by MongoDB ensure complete isolation as a document is updated; any errors cause the operation to roll back so that clients receive a consistent view of the document.

For situations that require atomicity of reads and writes to multiple documents (in a single or multiple collections), MongoDB supports distributed transactions, including transactions on replica sets and sharded clusters.

For more information, see transactions.

Does MongoDB handle caching?

Yes. MongoDB keeps most recently used data in RAM. If you have created indexes for your queries and your working data set fits in RAM, MongoDB serves all queries from memory.

MongoDB does not cache the query results in order to return the cached results for identical queries.

For more information on MongoDB and memory use, see WiredTiger and Memory Use.

How does MongoDB address SQL or Query injection?

BSONObj my_query = BSON( "name" << a_name );
auto_ptr<DBClientCursor> cursor = c.query("tutorial.persons", my_query);