$ifNull (expression operator)（表达式运算符）

Definition定义

$ifNull

The $ifNull expression evaluates input expressions for null values and returns:$ifNull表达式计算输入表达式中的null值并返回：

• The first non-null input expression value found.找到第一个非空输入表达式值。
• A replacement expression value if all input expressions evaluate to null.如果所有输入表达式的计算结果都为null，则返回替换表达式值。

$ifNull treats undefined values and missing fields as null.将未定义的值和缺少的字段视为null。

Compatibility兼容性

You can use $ifNull for deployments hosted in the following environments:您可以将$ifNull用于在以下环境中托管的部署：

• MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud：云中MongoDB部署的完全托管服务

• MongoDB Enterprise: The subscription-based, self-managed version of MongoDB：MongoDB的基于订阅的自我管理版本
• MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB：MongoDB的源代码可用、免费使用和自我管理版本

Syntax语法

{
   $ifNull: [
      <input-expression-1>,
      ...
      <input-expression-n>,
      <replacement-expression-if-null>
   ]
}

Examples示例

This inventory collection is used in the examples:此inventory集合用于以下示例：

db.inventory.insertMany( [
   { _id: 1, item: "buggy", description: "toy car", quantity: 300 },
   { _id: 2, item: "bicycle", description: null, quantity: 200 },
   { _id: 3, item: "flag" }
] )

db.inventory.aggregate(
   [
      {
         $project: {
            item: 1,
            description: { $ifNull: [ "$description", "Unspecified" ] }
         }
      }
   ]
)

{ _id: 1, item: "buggy", description: "toy car" }
{ _id: 2, item: "bicycle", description: "Unspecified" }
{ _id: 3, item: "flag", description: "Unspecified" }

db.inventory.aggregate(
   [
      {
         $project: {
            item: 1,
            value: { $ifNull: [ "$description", "$quantity", "Unspecified" ] }
         }
      }
   ]
)

{ _id: 1, item: "buggy", value: "toy car" }
{ _id: 2, item: "bicycle", value: 200 }
{ _id: 3, item: "flag", value: "Unspecified" }