$ifNull (aggregation)
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Definition
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$ifNull
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Evaluates an expression and returns the value of the expression if the expression evaluates to a non-null value. If the expression evaluates to a null value, including instances of undefined values or missing fields, returns the value of the replacement expression.
The
$ifNull
expression has the following syntax:The arguments can be any valid expression. For more information on expressions, see Expressions.
Example
The following example use a inventory
collection with the following documents:
The following operation uses the $ifNull
expression to return either the non-null description
field value or the string "Unspecified"
if the description
field is null or does not exist:
The operation returns the following results: