$ifNull (aggregation)
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Definition
$ifNull
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:{ $ifNull: [ <expression>, <replacement-expression-if-null> ] } 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:
{ "_id" : 1, "item" : "abc1", description: "product 1", qty: 300 } { "_id" : 2, "item" : "abc2", description: null, qty: 200 } { "_id" : 3, "item" : "xyz1", qty: 250 }
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:
db.inventory.aggregate( [ { $project: { item: 1, description: { $ifNull: [ "$description", "Unspecified" ] } } } ] )
The operation returns the following results:
{ "_id" : 1, "item" : "abc1", "description" : "product 1" } { "_id" : 2, "item" : "abc2", "description" : "Unspecified" } { "_id" : 3, "item" : "xyz1", "description" : "Unspecified" }