# $sum (aggregation)¶

## Definition¶

`$sum`

¶Calculates and returns the sum of numeric values.

`$sum`

ignores non-numeric values.`$sum`

is available in the following stages:`$group`

`$project`

`$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)`$match`

stage that includes an`$expr`

expression

In MongoDB 3.2 and earlier,

`$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:{ $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¶

### 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 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¶

### Use in `$group`

Stage¶

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¶

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.