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- $ifNull (aggregation)
$ifNull (aggregation)¶
- $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" }