Aggregation expressions can use bothuser-defined and system variables.
Variables can hold any BSON type data.To access the value of the variable, prefix the variable name withdouble dollar signs ($$); i.e. '$$<variable>'.
If the variable references an object, to access a specific field in theobject, use the dot notation; i.e. '$$<variable>.<field>'.
User Variables¶
User variable names can contain the ascii characters [_a-zA-Z0-9]and any non-ascii character.
User variable names must begin with a lowercase ascii letter [a-z]or a non-ascii character.
System Variables¶
MongoDB offers the following system variables:
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Description | |
|---|---|
A variable that returns the current datetime value. | |
| A variable that returns the current timestamp value.
|
| References the root document, i.e. the top-level document, currentlybeing processed in the aggregation pipeline stage. |
References the start of the field path being processed in theaggregation pipeline stage. Unless documented otherwise, allstages start with
| |
A variable which evaluates to the missing value. Allows for theconditional exclusion of fields. In a For an example of its usage, see Conditionally Exclude Fields. | |
| One of the allowed results of a $redact expression. |
One of the allowed results of a $redact expression. | |
One of the allowed results of a $redact expression. |
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Definition¶
$subtract¶
Mongodb Metabase
Subtracts two numbers to return the difference, or two dates toreturn the difference in milliseconds, or a date and a number inmilliseconds to return the resulting date.
The $subtract expression has the following syntax:
Metabase Oracle
The second argument is subtracted from the first argument.
The arguments can be any valid expression as long as they resolve to numbersand/or dates. To subtract a number from a date, the date must be thefirst argument. For more information on expressions, seeExpressions.
Examples¶
Consider a sales collection with the following documents:
Subtract Numbers¶
Metabase Mongodb Array
The following aggregation uses the $subtract expressionto compute the total by subtracting the discount from thesubtotal of price and fee.
The operation returns the following results:
Subtract Two Dates¶
The following aggregation uses the $subtract expressionto subtract $date from the current date, using the systemNOW (available starting in 4.2) and returns the difference inmilliseconds:
Alternatively, you can use the Date() for the current date:s
Both operations return documents that resemble the following:
Subtract Milliseconds from a Date¶
The following aggregation uses the $subtract expressionto subtract 5 * 60 * 1000 milliseconds (5 minutes) from the '$date'field:
The operation returns the following results:
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