# \$sum (aggregation)¶

## Definition¶

\$sum

Calculates and returns the sum of numeric values. \$sum ignores non-numeric values.

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 \$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: <expression> }

• \$sum has a list of specified expressions as its operand:

{ \$sum: [ <expression1>, <expression2> ... ]  }

## 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:

{ "_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 \$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.