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How to select a single field in MongoDB?

In my MongoDB, I have a student collection with 10 records having fields name and roll. One record of this collection is:

name

roll

{
"_id" : ObjectId("53d9feff55d6b4dd1171dd9e"),
"name" : "Swati",
"roll" : "80",
}


I want to retrieve the field "roll" only for all 10 records in the collections as we would do in traditional database by using:

SELECT roll FROM student


I went through many blogs but all are resulting in a query which must have WHERE clause in it. For example:

db.students.find({ "roll": { $gt: 70 })


The query is equivalent to:

SELECT * FROM student WHERE roll > 70


My requirement is to find one field only without any condition. So, what is the query operation for that.

Thanks in advance.

db.students.find({"roll": { $gt: 70})

SELECT roll FROM student WHERE roll > 70

13 Answers
13

From the MongoDB docs:

A projection can explicitly include several fields. In the following operation, find() method returns all documents that match the query. In the result set, only the item and qty fields and, by default, the _id field return in the matching documents.

db.inventory.find( { type: 'food' }, { item: 1, qty: 1 } )

In this example from the folks at Mongo, the returned documents will contain only the fields of item, qty, and _id.

item

qty

_id

Thus, you should be able to issue a statement such as:

db.student.find({}, {roll:1, _id:0})


The above statement will select all documents in the students collection, and the returned document will return only the roll field (and exclude the _id).

roll

_id

If we don't mention _id:0 the fields returned will be roll and _id. The '_id' field is always displayed by default. So we need to explicitly mention _id:0 along with roll.

_id:0

roll

_id

_id:0

roll

0

_id

{field_I_want:1, _id:0}

I think mattingly890 has the correct answer , here is another example along with the pattern/commmand

db.collection.find( {}, {your_key:1, _id:0})

db.collection.find( {}, {your_key:1, _id:0})

get all data from one field with _id

db.student.find({}, {roll:1})


SELECT roll FROM student

get all data from one field without _id

db.student.find({}, {roll:1, _id:0})


find specified data using where clause

db.student.find({roll: 80})


SELECT * FROM students WHERE roll = '80'

find a data using where clause and greater than condition

db.student.find({ "roll": { $gt: 70 })


SELECT * FROM student WHERE roll > '70'

Just for educational purposes you could also do it with any of the following ways:

1.

var query = {"roll": {$gt: 70};
var cursor = db.student.find(query);
cursor.project({"roll":1, "_id":0});


2.

var query = {"roll": {$gt: 70};
var projection = {"roll":1, "_id":0};
var cursor = db.student.find(query,projection);


`

Try the following query:

db.student.find({}, {roll: 1, _id: 0}).pretty();


Hope this helps!!

This works for me,

db.student.find({},{"roll":1})


no condition in where clause i.e., inside first curly braces.
inside next curly braces: list of projection field names to be needed in the result and 1 indicates particular field is the part of the query result

getting name of the student

student-details = db.students.find({{ "roll": {$gt: 70} },{"name": 1, "_id": False})


getting name & roll of the student

student-details = db.students.find({{ "roll": {$gt: 70}},{"name": 1,"roll":1,"_id": False})


If u want to retrieve the field "roll" only for all 10 records in the collections.
Then try this.

In MongoDb :

db.students.find( { } , { " roll " : { " $roll " })

In Sql :

select roll from students

db.student.find({}, {"roll":1, "_id":0})


This is equivalent to -

Select roll from student

db.student.find({}, {"roll":1, "name":1, "_id":0})

This is equivalent to -

Select roll, name from student

Use the Query like this in the shell:

1. Use database_name

database_name

e.g: use database_name


2. Which returns only assets particular field information when matched , _id:0 specifies not to display ID in the result

_id:0

db.collection_name.find( { "Search_Field": "value" },
{ "Field_to_display": 1,_id:0 } )


In mongodb 3.4 we can use below logic, i am not sure about previous versions

select roll from student ==> db.student.find(!{}, {roll:1})

the above logic helps to define some columns (if they are less)

For better understanding I have written similar MySQL query.

Selecting specific fields


MongoDB : db.collection_name.find({},{name:true,email:true,phone:true});

MySQL : SELECT name,email,phone FROM table_name;

Selecting specific fields with where clause


MongoDB : db.collection_name.find({email:'you@email.com'},{name:true,email:true,phone:true});

MySQL : SELECT name,email,phone FROM table_name WHERE email = 'you@email.com';

Here you go , 3 ways of doing , Shortest to boring :

db.student.find({}, 'roll _id'); // <--- Just multiple fields name space separated
// OR
db.student.find({}).select('roll _id'); // <--- Just multiple fields name space separated
// OR
db.student.find({}, {'roll' : 1 , '_id' : 1 ); // <---- Old lengthy boring way


To remove specific field use - operator :

-

db.student.find({}).select('roll -_id') // <--- Will remove id from result


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