MongoDB Documentation, Release 3.0.4 Query Behavior Query Statements Projections Chapter 3. MongoDB CRUD Operations 62 Queries in MongoDB return all fields in all matching documents by default. To limit the amount of data that MongoDB sends to applications, include a projection in the queries. By projecting results with a subset of fields, applications reduce their network overhead and processing requirements. In the diagram, the query selects documents from the users collection. Using a query selection operator to define the conditions for matching documents, the query selects documents that have age greater than (i.e. $gt ) 18 . Then the sort() modifier sorts the results by age in ascending order. For additional examples of queries, see Query Documents (page 96). Consider the following diagram of the query process that specifies a query criteria and a sort modifier: MongoDB provides a db.collection.findOne() method as a special case of find() that returns a single document. MongoDB queries exhibit the following behavior: • All queries in MongoDB address a single collection. • You can modify the query to impose limits , skips , and sort orders . • The order of documents returned by a query is not defined unless you specify a sort() . • Operations that modify existing documents (page 102) (i.e. updates ) use the same query syntax as queries to select documents to update. • In aggregation (page 439) pipeline, the $match pipeline stage provides access to MongoDB queries.