Sorting and Limiting Query Results in MongoDB

Review the following code, which demonstrates how to sort and limit query results.

Sorting Results

Use cursor.sort() to return query results in a specified order. Within the parentheses of  sort() , include an object that specifies the field(s) to sort by and the order of the sort. Use 1 for ascending order, and -1 for descending order.

Syntax:

db.collection.find(<query>).sort(<sort>)

Example:

// Return data on all music companies, sorted alphabetically from A to Z.

db.companies.find({ category_code: "music" }).sort({ name: 1 });

To ensure documents are returned in a consistent order, include a field that contains unique values in the sort. An easy way to do this is to include the 

_id field in the sort. 

Here's an example:

// Return data on all music companies, sorted alphabetically from A to Z. Ensure consistent sort order

db.companies.find({ category_code: "music" }).sort({ name: 1, _id: 1 });

Limiting Results

Use cursor.limit() to return query results in a specified order. Within the parentheses of  limit(), specify the maximum number of documents to return.

Syntax:

db.companies.find(<query>).limit(<number>)

Example:

// Return the three music companies with the highest number of employees. Ensure consistent sort order.

db.companies

  .find({ category_code: "music" })

  .sort({ number_of_employees: -1, _id: 1 })

  .limit(3);

Returning Specific Data from a Query in MongoDB

Review the following code, which demonstrates how to return selected fields from a query.

Add a Projection Document

To specify fields to include or exclude in the result set, add a projection document as the second parameter in the call to 

db.collection.find()

.Syntax:

db.collection.find( <query>, <projection> )

Include a Field

To include a field, set its value to 1 in the projection document.

Syntax:

db.collection.find( <query>, { <field> : 1 })

Example:

// Return all restaurant inspections - business name, result, and _id fields only

db.inspections.find(

  { sector: "Restaurant - 818" },

  { business_name: 1, result: 1 }

)

Exclude a Field

To exclude a field, set its value to 0 in the projection document.

Syntax:

db.collection.find(query, { <field> : 0, <field>: 0 })

Example:

// Return all inspections with result of "Pass" or "Warning" - exclude date and zip code

db.inspections.find(

  { result: { $in: ["Pass", "Warning"] } },

  { date: 0, "address.zip": 0 }

)

While the 

_id

 field is included by default, it can be suppressed by setting its value to 0 in any projection.

// Return all restaurant inspections - business name and result fields only

db.inspections.find(

  { sector: "Restaurant - 818" },

  { business_name: 1, result: 1, _id: 0 }

)

Counting Documents in a MongoDB Collection

Review the following code, which demonstrates how to count the number of documents that match a query.

Count Documents

Use db.collection.countDocuments() to count the number of documents that match a query. 

countDocuments() takes two parameters: a query document and an options document.

Syntax:

db.collection.countDocuments( <query>, <options> )

The query selects the documents to be counted.

Examples:

// Count number of docs in trip collection

db.trips.countDocuments({})

// Count number of trips over 120 minutes by subscribers

db.trips.countDocuments({ tripduration: { $gt: 120 }, usertype: "Subscriber" })