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$sum (aggregation)![](/docs/v4.4/assets/link.svg)
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Definition![](/docs/v4.4/assets/link.svg)
$sum
Calculates and returns the sum of numeric values.
$sum
ignores non-numeric values.$sum
is available in the following stages:$addFields
(Available starting in MongoDB 3.4)$set
(Available starting in MongoDB 4.2)$replaceRoot
(Available starting in MongoDB 3.4)$replaceWith
(Available starting in MongoDB 4.2)
Compatibility![](/docs/v4.4/assets/link.svg)
You can use $sum
for deployments hosted in the following
environments:
MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud
MongoDB Enterprise: The subscription-based, self-managed version of MongoDB
MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB
Syntax![](/docs/v4.4/assets/link.svg)
When used in the $group
stage, $sum
has the
following syntax and returns the collective sum of all the numeric
values that result from applying a specified expression to each
document in a group of documents that share the same group by key:
{ $sum: <expression> }
When used in the other supported stages, $sum
returns the
sum of the specified expression or list of expressions for each
document and has one of two syntaxes:
$sum
has one specified expression as its operand:{ $sum: <expression> } $sum
has a list of specified expressions as its operand:{ $sum: [ <expression1>, <expression2> ... ] }
For more information on expressions, see Expressions.
Behavior![](/docs/v4.4/assets/link.svg)
Result Data Type![](/docs/v4.4/assets/link.svg)
The result will have the same type as the input except when it cannot be represented accurately in that type. In these cases:
A 32-bit integer will be converted to a 64-bit integer if the result is representable as a 64-bit integer.
A 32-bit integer will be converted to a double if the result is not representable as a 64-bit integer.
A 64-bit integer will be converted to double if the result is not representable as a 64-bit integer.
Non-Numeric or Non-Existent Fields![](/docs/v4.4/assets/link.svg)
If used on a field that contains both numeric and non-numeric values,
$sum
ignores the non-numeric values and returns the sum of the
numeric values.
If used on a field that does not exist in any document in the collection,
$sum
returns 0
for that field.
If all operands are non-numeric, $sum
returns 0
.
Example | Field Values | Results |
---|---|---|
{ $sum : <field> } | Numeric | Sum of Values |
{ $sum : <field> } | Numeric and Non-Numeric | Sum of Numeric Values |
{ $sum : <field> } | Non-Numeric or Non-Existent | 0 |
Array Operand![](/docs/v4.4/assets/link.svg)
In the $group
stage, if the expression resolves to an
array, $sum
treats the operand as a non-numerical value.
In the other supported stages:
With a single expression as its operand, if the expression resolves to an array,
$sum
traverses into the array to operate on the numerical elements of the array to return a single value.With a list of expressions as its operand, if any of the expressions resolves to an array,
$sum
does not traverse into the array but instead treats the array as a non-numerical value.
Examples![](/docs/v4.4/assets/link.svg)
Use in $group
Stage![](/docs/v4.4/assets/link.svg)
Consider a sales
collection with the following documents:
{ "_id" : 1, "item" : "abc", "price" : 10, "quantity" : 2, "date" : ISODate("2014-01-01T08:00:00Z") } { "_id" : 2, "item" : "jkl", "price" : 20, "quantity" : 1, "date" : ISODate("2014-02-03T09:00:00Z") } { "_id" : 3, "item" : "xyz", "price" : 5, "quantity" : 5, "date" : ISODate("2014-02-03T09:05:00Z") } { "_id" : 4, "item" : "abc", "price" : 10, "quantity" : 10, "date" : ISODate("2014-02-15T08:00:00Z") } { "_id" : 5, "item" : "xyz", "price" : 5, "quantity" : 10, "date" : ISODate("2014-02-15T09:05:00Z") }
Grouping the documents by the day and the year of the date
field,
the following operation uses the $sum
accumulator to compute the
total amount and the count for each group of documents.
db.sales.aggregate( [ { $group: { _id: { day: { $dayOfYear: "$date"}, year: { $year: "$date" } }, totalAmount: { $sum: { $multiply: [ "$price", "$quantity" ] } }, count: { $sum: 1 } } } ] )
The operation returns the following results:
{ "_id" : { "day" : 46, "year" : 2014 }, "totalAmount" : 150, "count" : 2 } { "_id" : { "day" : 34, "year" : 2014 }, "totalAmount" : 45, "count" : 2 } { "_id" : { "day" : 1, "year" : 2014 }, "totalAmount" : 20, "count" : 1 }
Using $sum
on a non-existent field returns a value of 0
.
The following operation attempts to $sum
on qty
:
db.sales.aggregate( [ { $group: { _id: { day: { $dayOfYear: "$date"}, year: { $year: "$date" } }, totalAmount: { $sum: "$qty" }, count: { $sum: 1 } } } ] )
The operation returns:
{ "_id" : { "day" : 46, "year" : 2014 }, "totalAmount" : 0, "count" : 2 } { "_id" : { "day" : 34, "year" : 2014 }, "totalAmount" : 0, "count" : 2 } { "_id" : { "day" : 1, "year" : 2014 }, "totalAmount" : 0, "count" : 1 }
Use in $project
Stage![](/docs/v4.4/assets/link.svg)
A collection students
contains the following documents:
{ "_id": 1, "quizzes": [ 10, 6, 7 ], "labs": [ 5, 8 ], "final": 80, "midterm": 75 } { "_id": 2, "quizzes": [ 9, 10 ], "labs": [ 8, 8 ], "final": 95, "midterm": 80 } { "_id": 3, "quizzes": [ 4, 5, 5 ], "labs": [ 6, 5 ], "final": 78, "midterm": 70 }
The following example uses the $sum
in the
$project
stage to calculate the total quiz scores, the
total lab scores, and the total of the final and the midterm:
db.students.aggregate([ { $project: { quizTotal: { $sum: "$quizzes"}, labTotal: { $sum: "$labs" }, examTotal: { $sum: [ "$final", "$midterm" ] } } } ])
The operation results in the following documents:
{ "_id" : 1, "quizTotal" : 23, "labTotal" : 13, "examTotal" : 155 } { "_id" : 2, "quizTotal" : 19, "labTotal" : 16, "examTotal" : 175 } { "_id" : 3, "quizTotal" : 14, "labTotal" : 11, "examTotal" : 148 }
In the other supported stages:
With a single expression as its operand, if the expression resolves to an array,
$sum
traverses into the array to operate on the numerical elements of the array to return a single value.With a list of expressions as its operand, if any of the expressions resolves to an array,
$sum
does not traverse into the array but instead treats the array as a non-numerical value.