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$subtract (aggregation)

$subtract

Subtracts two numbers to return the difference, or two dates to return the difference in milliseconds, or a date and a number in milliseconds to return the resulting date.

The $subtract expression has the following syntax:

{ $subtract: [ <expression1>, <expression2> ] }

The second argument is subtracted from the first argument.

The arguments can be any valid expression as long as they resolve to numbers and/or dates. To subtract a number from a date, the date must be the first argument. For more information on expressions, see Expressions.

Starting in MongoDB 4.4.3, 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.

Consider a sales collection with the following documents:

db.sales.insertMany([
{ "_id" : 1, "item" : "abc", "price" : 10, "fee" : 2, "discount" : 5, "date" : ISODate("2014-03-01T08:00:00Z") },
{ "_id" : 2, "item" : "jkl", "price" : 20, "fee" : 1, "discount" : 2, "date" : ISODate("2014-03-01T09:00:00Z") }
])

The following aggregation uses the $subtract expression to compute the total by subtracting the discount from the subtotal of price and fee.

db.sales.aggregate( [ { $project: { item: 1, total: { $subtract: [ { $add: [ "$price", "$fee" ] }, "$discount" ] } } } ] )

The operation returns the following results:

{ "_id" : 1, "item" : "abc", "total" : 7 }
{ "_id" : 2, "item" : "jkl", "total" : 19 }

The following aggregation uses the $subtract expression to subtract $date from the current date, using the system NOW (available starting in 4.2) and returns the difference in milliseconds:

db.sales.aggregate( [ { $project: { item: 1, dateDifference: { $subtract: [ "$$NOW", "$date" ] } } } ] )

Alternatively, you can use the Date() for the current date:s

db.sales.aggregate( [ { $project: { item: 1, dateDifference: { $subtract: [ new Date(), "$date" ] } } } ] )

Both operations return documents that resemble the following:

{ "_id" : 1, "item" : "abc", "dateDifference" : NumberLong("186136746187") }
{ "_id" : 2, "item" : "jkl", "dateDifference" : NumberLong("186133146187") }

The following aggregation uses the $subtract expression to subtract 5 * 60 * 1000 milliseconds (5 minutes) from the "$date" field:

db.sales.aggregate( [ { $project: { item: 1, dateDifference: { $subtract: [ "$date", 5 * 60 * 1000 ] } } } ] )

The operation returns the following results:

{ "_id" : 1, "item" : "abc", "dateDifference" : ISODate("2014-03-01T07:55:00Z") }
{ "_id" : 2, "item" : "jkl", "dateDifference" : ISODate("2014-03-01T08:55:00Z") }
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