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$exists

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Definition定义

$exists

Syntax: { 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，则查询仅返回不包含该字段的文档。[1]

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

Tip

See also: 另请参阅：

[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.若要查询null或缺少的字段，请参阅查询null或丢失的字段。

Examples实例

Exists and Not Equal To存在但不等于

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空值

The following examples uses a collection named spices with the following documents:以下示例使用了一个名为spices的集合，其中包含以下文档：

db.spices.insertMany( [
   { saffron: 5, cinnamon: 5, mustard: null },
   { saffron: 3, cinnamon: null, mustard: 8 },
   { saffron: null, cinnamon: 3, mustard: 9 },
   { saffron: 1, cinnamon: 2, mustard: 3 },
   { saffron: 2, mustard: 5 },
   { saffron: 3, cinnamon: 2 },
   { saffron: 4 },
   { cinnamon: 2, mustard: 4 },
   { cinnamon: 2 },
   { mustard: 6 }
] )

$exists: true

The following query specifies the query predicate saffron: { $exists: true }:以下查询指定查询谓词saffron: { $exists: true }：

db.spices.find( { saffron: { $exists: true } } )

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

{ saffron: 5, cinnamon: 5, mustard: null }
{ saffron: 3, cinnamon: null, mustard: 8 }
{ saffron: null, cinnamon: 3, mustard: 9 }
{ saffron: 1, cinnamon: 2, mustard: 3 }
{ saffron: 2, mustard: 5 }
{ saffron: 3, cinnamon: 2 }
{ saffron: 4 }

$exists: false

The following query specifies the query predicate cinnamon: { $exists: false }:以下查询指定查询谓词cinnamon: { $exists: false }：

db.spices.find( { cinnamon: { $exists: false } } )

The results consist of those documents that do not contain the field cinnamon:结果包括那些不包含字段cinnamon: { $exists: false }的文件：

{ saffron: 2, mustard: 5 }
{ saffron: 4 }
{ mustard: 6 }

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

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

The following table compares $exists query performance using sparse and non-sparse indexes:下表比较了使用稀疏索引和非稀疏索引的$exists查询性能：

$exists Query查询Using a Sparse Index使用稀疏索引Using a Non-Sparse Index使用非稀疏索引
{ $exists: true }Most efficient. MongoDB can make an exact match and does not require a FETCH.效率最高。MongoDB可以进行精确匹配，并且不需要FETCH。More efficient than queries without an index, but still requires a FETCH.比没有索引的查询更高效，但仍然需要FETCH。
{ $exists: false }Cannot use the index and requires a COLLSCAN.无法使用索引，需要COLLSCAN。Requires a FETCH.需要FETCH。

Queries that use { $exists: true } on fields that use a non-sparse index or that use { $exists: true } on fields that are not indexed examine all documents in a collection. 在使用非稀疏索引的字段上使用{ $exists: true }的查询，或在未编制索引的字段中使用{ $exists: true }的询问，都会检查集合中的所有文档。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: "MDB", auditDate: new Date( "2021-05-18T16:12:23Z" ) },
       { _id: 1, symbol: "MDB", auditDate: new Date( "2021-04-21T11:34:45Z" ) },
       { _id: 2, symbol: "MSFT", auditDate: new Date( "2021-02-24T15:11:32Z" ) },
       { _id: 3, symbol: "MSFT", auditDate: null },
       { _id: 4, symbol: "MSFT", auditDate: new Date( "2021-07-13T18:32:54Z" ) },
       { _id: 5, symbol: "AAPL" }
    ] )

    The document with an _id of:_id为以下情况的文档：

    • 3 has a null auditDate value.具有值为null的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null）并使用稀疏索引的文档：

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

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

Tip

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

  • Can use $ne: null instead of $exists: true.可以使用$ne:null而不是$exist: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. Documents that are missing the auditDate value or have a null auditDate value are not counted.该示例返回4。缺少auditDate值或auditDate为空的文档不计算在内。

Tip

See also: 另请参阅：

Query for Null or Missing Fields查询Null或缺少的字段