MongoDB Projection Operator - $slice

Description

In MongoDB, the $slice operator decide the retrieval of a number of items of an array that a query returns. The $slice accepts a number including a negative number as an argument and returns the output. The argument can be the form of skip and limit, that means the $slice operator retrieve the number of items specified in the limit from the array to skip a specific of a number of items.

Syntax:

db.collection.find( { field: value }, { array: {$slice: count| [skip, limit] } } );

Parameters:

Our database name is 'myinfo' and our collection name is 'table2'. Here, is the collection bellow.

Sample collection "table2"

{
        "_id" : ObjectId("52860b2cf313832c9a6f1018"),
        "student_id" : "STU001",
        "sem" : "sem1",
        "score" : [
                45,
                59,
                75,
                67,
                52,
                72
        ]
}
{
        "_id" : ObjectId("52860b81f313832c9a6f1019"),
        "student_id" : "STU002",
        "sem" : "sem1",
        "score" : [
                55,
                69,
                57,
                76,
                63,
                49
        ]
}

Example - 1 of MongoDB Projection Operator - $slice

If we want to select all documents from the collection "table2" which satisfying the conditions -

The student_id is "STU001"

Display the score of first 3 subjects

the following mongodb command can be used :

>db.table2.find({ "student_id":"STU001" }, { "score": { $slice: 3}}).pretty();

N.B. find() method displays the documents in a non structured format but to display the results in a formatted way, the pretty() method can be used.

Output:

{
        "_id" : ObjectId("52860b2cf313832c9a6f1018"),
        "student_id" : "STU001",
        "sem" : "sem1",
        "score" : [
                45,
                59,
                75
        ]
}

Example - 2 of MongoDB Projection Operator - $slice

If we want to select all documents from the collection "table2" which satisfying the conditions -

display the score of first 3 subjects for all students.

the following mongodb command can be used :

>db.table2.find({}, { "score": { $slice: 3}}).pretty();

N.B. find() method displays the documents in a non structured format but to display the results in a formatted way, the pretty() method can be used.

Output:

{
        "_id" : ObjectId("52860b2cf313832c9a6f1018"),
        "student_id" : "STU001",
        "sem" : "sem1",
        "score" : [
                45,
                59,
                75
        ]
}
{
        "_id" : ObjectId("52860b81f313832c9a6f1019"),
        "student_id" : "STU002",
        "sem" : "sem1",
        "score" : [
                55,
                69,
                57
        ]
}

Example - 3 of MongoDB Projection Operator - $slice

If we want to select all documents from the collection "table1" which satisfying the conditions -

Display the score of fourth and fifth subject for all the students.

the following mongodb command can be used :

>db.table2.find({}, { "score": { $slice: [3,2]}}).pretty();

N.B. find() method displays the documents in a non structured format but to display the results in a formatted way, the pretty() method can be used.

Output:

{
        "_id" : ObjectId("52860b2cf313832c9a6f1018"),
        "student_id" : "STU001",
        "sem" : "sem1",
        "score" : [
                67,
                52
        ]
}
{
        "_id" : ObjectId("52860b81f313832c9a6f1019"),
        "student_id" : "STU002",
        "sem" : "sem1",
        "score" : [
                76,
                63
        ]
}

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