$subtract (aggregation)
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Definition
$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
$subtractexpression 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.
Examples
Consider a sales collection with the following documents:
{ "_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") }
Subtract Numbers
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 }
Subtract Two Dates
The following aggregation uses the $subtract expression to subtract $date from the current date and returns the difference in milliseconds:
db.sales.aggregate( [ { $project: { item: 1, dateDifference: { $subtract: [ new Date(), "$date" ] } } } ] )
The operation returns the following results:
{ "_id" : 1, "item" : "abc", "dateDifference" : NumberLong("11713985194") }
{ "_id" : 2, "item" : "jkl", "dateDifference" : NumberLong("11710385194") }
Subtract Milliseconds from a Date
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") }