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Variables
Aggregation expressions can use both user-defined and system variables.
Variables can hold any BSON type data. To access the value of the variable, prefix the variable name with double dollar signs (
$); i.e. "$
".
For example:
Variable
Access via $Brief Description close $CLOSE
returns the absolute value of a number
context
$CONTEXT returns a numeric value between 0 and π radians for x between -1 and 1 cursor $CURSOR
returns the inverse hyperbolic cosine of a number
date
$DATE N/A env $ENV
N/A
now
$NOW A variable that returns the current datetime value random $RANDOM
returns a floating-point, pseudo-random number in the range 0 to less than 1
remove
$REMOVE A variable which evaluates to the missing value request $REQUEST
N/A
root
$ROOT References the root currently being processed in the aggregation pipeline stage script $SCRIPT
N/A
var
\$VAR
N/A