$exists

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Definition定义

$exists

Syntax: 语法：{ field: { $exists: <boolean> } }

When <boolean> is true, $exists matches the documents that contain the field, including documents where the field value is null. 当<boolean>为true时，$exists匹配包含该字段的文档，包括字段值为null的文档。If <boolean> is false, the query returns only the documents that do not contain the field. 如果<boolean>为false，则查询只返回不包含该字段的文档。[1]

MongoDB $exists does not correspond to SQL operator exists. MongoDB$exists与SQL运算符exists不对应。For SQL exists, refer to the $in operator.对于SQL exists，请参阅$in运算符。

Tip提示
See also: 参阅：
[1] Starting in MongoDB 4.2, users can no longer use the query filter $type: 0 as a synonym for $exists:false. 从MongoDB 4.2开始，用户不能再使用查询筛选器$type:0作为$exists:false的同义词。To query for null or missing fields, see Query for Null or Missing Fields.要查询空字段或缺失字段，请参阅查询空字段和缺失字段。

Examples示例

Exists and Not Equal To存在且不等于

Consider the following example:考虑以下示例：

db.inventory.find( { qty: { $exists: true, $nin: [ 5, 15 ] } } )

This query will select all documents in the inventory collection where the qty field exists and its value does not equal 5 or 15.此查询将选择inventory集合中存在qty字段且其值不等于5或15的所有单据。

Null Values值

The following examples uses a collection named records with the following documents:以下示例使用具有以下文档的名为records的集合：

{ a: 5, b: 5, c: null }
{ a: 3, b: null, c: 8 }
{ a: null, b: 3, c: 9 }
{ a: 1, b: 2, c: 3 }
{ a: 2, c: 5 }
{ a: 3, b: 2 }
{ a: 4 }
{ b: 2, c: 4 }
{ b: 2 }
{ c: 6 }

$exists: true

The following query specifies the query predicate a: { $exists: true }:以下查询指定了查询谓词a: { $exists: true }：

db.records.find( { a: { $exists: true } } )

The results consist of those documents that contain the field a, including the document whose field a contains a null value:结果由包含字段a的文档组成，包括字段a包含空值的文档：

{ a: 5, b: 5, c: null }
{ a: 3, b: null, c: 8 }
{ a: null, b: 3, c: 9 }
{ a: 1, b: 2, c: 3 }
{ a: 2, c: 5 }
{ a: 3, b: 2 }
{ a: 4 }

$exists: false

The following query specifies the query predicate b: { $exists: false }:以下查询指定了查询谓词b: { $exists: false }：

db.records.find( { b: { $exists: false } } )

The results consist of those documents that do not contain the field b:结果包括不包含字段b的文档：

{ a: 2, c: 5 }
{ a: 4 }
{ c: 6 }

Starting in MongoDB 4.2, users can no longer use the query filter $type: 0 as a synonym for $exists:false. 从MongoDB 4.2开始，用户不能再使用查询筛选器$type:0作为$exists:false的同义词。To query for null or missing fields, see Query for Null or Missing Fields.要查询空字段或缺失字段，请参阅查询空字段和缺失字段。

Use a Sparse Index to Improve $exists Performance使用稀疏索引提高$exists性能

The following scenario is not optimal because all of the collection's documents are examined:以下场景不是最佳的，因为集合的所有文档都已检查：

  • You use a query to retrieve or count documents, and使用查询检索或统计文档，以及
  • use field: { $exists: true }, and使用field: { $exists: true }，并且
  • the field has a non-sparse index or does not have an index.field具有非稀疏索引或没有索引。

To improve performance, create a sparse index on the field as shown in the following scenario:要提高性能，请在field上创建稀疏索引，如以下场景所示：

  1. Create a stockSales collection:创建stockSales集合：

    db.stockSales.insertMany( [
   { _id: 0, symbol: "ABC", auditDate: new Date( "2021-05-18T16:12:23Z" ) },
   { _id: 1, symbol: "ABC", auditDate: new Date( "2021-04-21T11:34:45Z" ) },
   { _id: 2, symbol: "DEF", auditDate: new Date( "2021-02-24T15:11:32Z" ) },
   { _id: 3, symbol: "DEF", auditDate: null },
   { _id: 4, symbol: "DEF", auditDate: new Date( "2021-07-13T18:32:54Z" ) },
   { _id: 5, symbol: "XYZ" }
] )

    The document with an _id of:_id为的文档：

    • 3 has a null auditDate value.具有空auditDate值。
    • 5 is missing the auditDate value.缺少auditDate值。

  2. Create a sparse index on the auditDate field:在auditDate字段上创建稀疏索引：

    db.getCollection( "stockSales" ).createIndex(
   { auditDate: 1 },
   { name: "auditDateSparseIndex", sparse: true }
)

  3. The following example counts the documents where the auditDate field has a value (including null) and uses the sparse index:以下示例统计auditDate字段具有值（包括空）并使用稀疏索引的文档：

    db.stockSales.countDocuments( { auditDate: { $exists: true } } )

    The example returns 5. 示例返回5。The document that is missing the auditDate value is not counted.不计算缺少auditDate值的文档。

Tip提示

If you only need documents where the field has a non-null value, you:如果您只需要field具有非空值的文档，您可以：

  • Can use $ne: null instead of $exists: true.可以使用$ne:null而不是$exists:true。
  • Do not need a sparse index on the field.不需要field上的稀疏索引。

For example, using the stockSales collection:例如，使用stockSales集合：

db.stockSales.countDocuments( { auditDate: { $ne: null } } )

The example returns 4. 示例返回4。Documents that are missing the auditDate value or have a null auditDate value are not counted.缺少auditDate值或auditDate值为空的文档不计算在内。

Tip提示
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