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$subtract

Definition

Subtracts numbers, subtracts a millisecond offset from a date, or returns the millisecond difference between two dates.

Expression operator · Arithmetic · Studio Visual: available. Studio validates the complete expression separately.

Syntax

{ "$subtract": [ <left>, <right> ] }

Replace angle-bracket placeholders with expressions or values.

Optional parameters may be omitted.

Parameters

Parameter Required Description
left Yes Expression to subtract from.
right Yes Expression to subtract.

Arguments are positional: keep the order shown. The authoring signature accepts 2–2 arguments.

Behavior

The following observations use the example expression below. Each row substitutes one argument while retaining the other example arguments. A missing field differs from an explicit null; the result shown is what Core actually returned.

For the left argument:

Input case Supplied value Result
null null null
missing Field absent null
decimal 2.5 0.5 (double)
numeric string "12" Error — $subtract only supports numeric types.
text "hello" Error — $subtract only supports numeric types.
boolean true Error — $subtract only supports numeric types.
typed date {"$date":"2026-01-15T15:00:00Z"} {'$numberLong': '1768489199998'} (date)
empty array [] Error — $subtract only supports numeric types.
numeric array [1,2,3] Error — $subtract only supports numeric types.
object {"x":1} Error — $subtract only supports numeric types.

Argument handling

Invocation Result
{"$subtract":[]} Error — $subtract requires exactly two arguments.
{"$subtract":{"unexpected":1}} Error — $subtract requires exactly two arguments.

Examples

Expression: 

{
  "$subtract": [
    5,
    2
  ]
}

Input document: 

{}

Observed SrotaX result: 3.0 (double).

Detailed input cases

Expand the complete argument-by-argument test matrix

Each table changes only the indicated argument from the example above. Other arguments retain their baseline values. Values normally come from the input field probe; literal configuration arguments are replaced directly. “Missing” means the field is absent, not explicitly null. Numeric strings are strings, not numbers.

Argument: left[0]

Input case Supplied value SrotaX result
null null null
missing Field absent null
zero 0 -2.0 (double)
negative -2 -4.0 (double)
decimal 2.5 0.5 (double)
numeric string "12" Error — $subtract only supports numeric types.
text "hello" Error — $subtract only supports numeric types.
empty string "" Error — $subtract only supports numeric types.
boolean true Error — $subtract only supports numeric types.
false false Error — $subtract only supports numeric types.
typed date {"$date":"2026-01-15T15:00:00Z"} {'$numberLong': '1768489199998'} (date)
decimal128 {"$numberDecimal":"1.25"} -0.75 (double)
empty array [] Error — $subtract only supports numeric types.
numeric array [1,2,3] Error — $subtract only supports numeric types.
mixed array [1,null,"x",3] Error — $subtract only supports numeric types.
nested array [[1,2],[3]] Error — $subtract only supports numeric types.
empty object {} Error — $subtract only supports numeric types.
object {"x":1} Error — $subtract only supports numeric types.
ISO date string "2026-01-15T15:00:00Z" Error — $subtract only supports numeric types.

Argument: right[1]

Input case Supplied value SrotaX result
null null null
missing Field absent null
zero 0 5.0 (double)
negative -2 7.0 (double)
decimal 2.5 2.5 (double)
numeric string "12" Error — $subtract only supports numeric types.
text "hello" Error — $subtract only supports numeric types.
empty string "" Error — $subtract only supports numeric types.
boolean true Error — $subtract only supports numeric types.
false false Error — $subtract only supports numeric types.
typed date {"$date":"2026-01-15T15:00:00Z"} Error — $subtract only supports numeric types.
decimal128 {"$numberDecimal":"1.25"} 3.75 (double)
empty array [] Error — $subtract only supports numeric types.
numeric array [1,2,3] Error — $subtract only supports numeric types.
mixed array [1,null,"x",3] Error — $subtract only supports numeric types.
nested array [[1,2],[3]] Error — $subtract only supports numeric types.
empty object {} Error — $subtract only supports numeric types.
object {"x":1} Error — $subtract only supports numeric types.
ISO date string "2026-01-15T15:00:00Z" Error — $subtract only supports numeric types.

Verification

This page records 41 executed cases. It covers individual argument substitutions, not every cross-product of arguments, locale, daylight-saving transitions, numeric overflow, collation or resource limits. No untested behavior is guaranteed.

Download exact expressions, inputs and SrotaX results. Errors are observed messages, not stable public error codes. The reference generator reads the checked-in execution report; do not edit generated observations by hand.

MongoDB differences

Compared with MongoDB 9.0.4: 7 exact BSON matches, 27 both rejected, 7 value/type/acceptance differences, 0 object-field-order differences.

“Exact” includes numeric BSON types and object key order. A value of 3 as int32 differs from 3.0 as double. Object-field-order differences are reported separately; object key ordering is not enforced by the engine regression baseline. Both rejected does not mean the error codes or messages match. This is finite-case evidence, not full compatibility certification.

Show observed differences
Tested case SrotaX MongoDB
all / baseline 3.0 (double) 3 (int32)
left[0] / zero -2.0 (double) -2 (int32)
left[0] / negative -4.0 (double) -4 (int32)
left[0] / decimal128 -0.75 (double) -0.75 (decimal128)
right[1] / zero 5.0 (double) 5 (int32)
right[1] / negative 7.0 (double) 7 (int32)
right[1] / decimal128 3.75 (double) 3.75 (decimal128)

Download full comparison evidence.