$sum (aggregation)
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Definition
-
$sum
-
Calculates and returns the sum of numeric values.
$sum
ignores non-numeric values.Changed in version 3.2:
$sum
is available in the$group
and$project
stages. In previous versions of MongoDB,$sum
is available in the$group
stage only.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:When used in the
$project
stage,$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
has a list of specified expressions as its operand:
For more information on expressions, see Expressions.
Behavior
Non-Numeric or Non-Existent Fields
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
In the $group
stage, if the expression resolves to an array, $sum
treats the operand as a non-numerical value.
In the $project
stage:
- 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
Use in $group
Stage
Consider a sales
collection with the following documents:
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.
The operation returns the following results:
Using $sum
on a non-existent field returns a value of 0
. The following operation attempts to $sum
on qty
:
The operation returns:
Use in $project
Stage
A collection students
contains the following documents:
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:
The operation results in the following documents:
In the $project
stage:
- 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.