Specify a Query

Overview

In this guide, you can learn how to specify a query by using the C++ driver.

You can refine the set of documents that a query returns by creating a query filter. A query filter is an expression that specifies the search criteria that MongoDB uses to match documents in a read or write operation. In a query filter, you can prompt the driver to search for documents that have an exact match to your query, or you can compose query filters to express more complex matching criteria.

mongocxx::uri uri("<connection string>");
mongocxx::client client(uri);
auto db = client["db"];
auto collection = db["fruits"];
std::vector<bsoncxx::document::value> fruits;
fruits.push_back(make_document(kvp("_id", 1), kvp("name", "apples"), kvp("qty", 5), kvp("rating", 3), kvp("color", "red"), kvp("type", make_array("fuji", "honeycrisp"))));
fruits.push_back(make_document(kvp("_id", 2), kvp("name", "bananas"), kvp("qty", 7), kvp("rating", 4), kvp("color", "yellow"), kvp("type", make_array("cavendish"))));
fruits.push_back(make_document(kvp("_id", 3), kvp("name", "oranges"), kvp("qty", 6), kvp("rating", 2), kvp("type", make_array("naval", "mandarin"))));
fruits.push_back(make_document(kvp("_id", 4), kvp("name", "pineapples"), kvp("qty", 3), kvp("rating", 5), kvp("color", "yellow")));
auto result = collection.insert_many(fruits);

Exact Match

Literal value queries return documents that have an exact match to your query filter.

The following example specifies a query filter as a parameter to the find() method. The code returns all documents in which the value of the color field is "yellow":

auto cursor = collection.find(make_document(kvp("color", "yellow")));
for (auto &&doc : cursor)
{
    std::cout << bsoncxx::to_json(doc) << std::endl;
}

{ "_id" : 2, "name" : "bananas", "qty" : 7, "rating" : 4, "color" : "yellow", "type" : [ "cavendish" ] }
{ "_id" : 4, "name" : "pineapple", "qty" : 3, "rating" : 5, "color" : "yellow" }

auto cursor = collection.find({});

Comparison Operators

Comparison operators evaluate a document field value against a specified value in your query filter. The following list defines common comparison operators:

• $gt: Greater than

• $lte: Less than or Equal

• $ne: Not equal

To view a full list of comparison operators, see the Comparison Query Operators guide in the MongoDB Server manual.

The following example specifies a comparison operator in a query filter as a parameter to the find() method. The code returns all documents that have a rating field value greater than 2:

auto cursor = collection.find(make_document(kvp("rating", make_document(kvp("$gt", 2)))));
for(auto&& doc : cursor) {
    std::cout << bsoncxx::to_json(doc) << std::endl;
}

{ "_id" : 1, "name" : "apples", "qty" : 5, "rating" : 3, "color" : "red", "type" : [ "fuji", "honeycrisp" ] }
{ "_id" : 2, "name" : "bananas", "qty" : 7, "rating" : 4, "color" : "yellow", "type" : [ "cavendish" ] }
{ "_id" : 4, "name" : "pineapples", "qty" : 3, "rating" : 5, "color" : "yellow" }

Logical Operators

Logical operators match documents by using logic applied to the results of two or more sets of expressions. The following list describes each logical operator:

• $and: Returns all documents that match the conditions of all clauses

• $or: Returns all documents that match the conditions of one clause

• $nor: Returns all documents that do not match the conditions of any clause

• $not: Returns all documents that do not match the expression

To learn more about logical operators, see the Logical Query Operators guide in the MongoDB Server manual.

The following example specifies a logical operator in a query filter as a parameter to the find() method. The code returns all documents in which the qty field value is greater than 5 or the color field value is "yellow":

auto cursor = collection.find(
    make_document(kvp("$or",
                        make_array(make_document(kvp("qty", make_document(kvp("$gt", 5)))),
                                    make_document(kvp("color", "yellow"))))));
for (auto&& doc : cursor) {
    std::cout << bsoncxx::to_json(doc) << std::endl;
}

{ "_id" : 2, "name" : "bananas", "qty" : 7, "rating" : 4, "color" : "yellow", "type" : [ "cavendish" ] }
{ "_id" : 3, "name" : "oranges", "qty" : 6, "rating" : 2, "type" : [ "naval", "mandarin" ] }
{ "_id" : 4, "name" : "pineapples", "qty" : 3, "rating" : 5, "color" : "yellow" }

Array Operators

Array operators match documents based on the value or quantity of elements in an array field. The following list describes the available array operators:

• $all: Returns documents with arrays that contain all elements in the query

• $elemMatch: Returns documents if an element in their array field matches all conditions in the query

• $size: Returns all documents with arrays of a specified size

To learn more about array operators, see the Array Query Operators guide in the MongoDB Server manual.

The following example specifies an array operator in a query filter as a parameter to the find() method. The code returns all documents in which the type array field contains 2 elements:

auto cursor = collection.find(make_document(kvp("type", make_document(kvp("$size", 2)))));
for(auto&& doc : cursor) {
    std::cout << bsoncxx::to_json(doc) << std::endl;
}

{ "_id" : 1, "name" : "apples", "qty" : 5, "rating" : 3, "color" : "red", "type" : [ "fuji", "honeycrisp" ] }
{ "_id" : 3, "name" : "oranges", "qty" : 6, "rating" : 2, "type" : [ "naval", "mandarin" ] }

Element Operators

Element operators query data based on the presence or type of a field.

To learn more about element operators, see the Element Query Operators guide in the MongoDB Server manual.

The following example specifies an element operator in a query filter as a parameter to the find() method. The code returns all documents that have a color field:

auto cursor = collection.find(make_document(kvp("color", make_document(kvp("$exists", true)))));
for(auto&& doc : cursor) {
    std::cout << bsoncxx::to_json(doc) << std::endl;
}

{ "_id" : 1, "name" : "apples", "qty" : 5, "rating" : 3, "color" : "red", "type" : [ "fuji", "honeycrisp" ] }
{ "_id" : 2, "name" : "bananas", "qty" : 7, "rating" : 4, "color" : "yellow", "type" : [ "cavendish" ] }
{ "_id" : 4, "name" : "pineapples", "qty" : 3, "rating" : 5, "color" : "yellow" }

Evaluation Operators

Evaluation operators return data based on evaluations of either individual fields or the entire collection's documents.

The following list describes common evaluation operators:

• $text: Performs a text search on the documents

• $regex: Returns documents that match a specified regular expression

• $mod: Performs a modulo operation on the value of a field and returns documents where the remainder is a specified value

To view a full list of evaluation operators, see the Evaluation Query Operators guide in the MongoDB Server manual.

The following example specifies an evaluation operator in a query filter as a parameter to the find() method. The code uses a regular expression to return all documents in which the name field value has at least two consecutive "p" characters:

auto cursor = collection.find(make_document(kvp("name", make_document(kvp("$regex", "p{2,}")))));
for(auto&& doc : cursor) {
    std::cout << bsoncxx::to_json(doc) << std::endl;
}

{ "_id" : 1, "name" : "apples", "qty" : 5, "rating" : 3, "color" : "red", "type" : [ "fuji", "honeycrisp" ] }
{ "_id" : 4, "name" : "pineapples", "qty" : 3, "rating" : 5, "color" : "yellow" }

Additional Information

To learn more about querying documents, see the Query Documents guide in the MongoDB Server manual.

To learn more about retrieving documents with the C++ driver, see the Retrieve Data guide.