MongoDB - MongoDB CRUD Operations, Query Documents, Query for Null or Missing Fields
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values differently.
The examples on this page use the db.collection.find() method in the mongo shell. To populate the users collection referenced in the examples, run the following in mongo shell:
db.users.insert(
[
{ "_id" : 900, "name" : null },
{ "_id" : 901 }
]
)
Equality Filter
The { name : null } query matches documents that either contain the name field whose value is null or that do not contain the name field.
Given the following query:
db.users.find( { name: null } )
The query returns both documents:
{ "_id" : 900, "name" : null }
{ "_id" : 901 }
If the query uses an index that is sparse, however, then the query will only match null values, not missing fields.
Changed in version 2.6: If using the sparse index results in an incomplete result, MongoDB will not use the index unless a hint() explicitly specifies the index. See Sparse Indexes for more information.
Type Check
The { name : { $type: 10 } } query matches documents that contains the name field whose value is null only; i.e. the value of the item field is of BSON Type Null (i.e. 10) :
db.users.find( { name : { $type: 10 } } )
The query returns only the document where the item field has a null value:
{ "_id" : 900, "name" : null }
Existence Check
The { name : { $exists: false } } query matches documents that do not contain the item field:
db.users.find( { name : { $exists: false } } )
The query returns only the document that does not contain the item field:
{ "_id" : 901 }
SEE ALSO: The reference documentation for the $type and $exists operators.
MongoDB - MongoDB CRUD Operations, Query Documents, Query for Null or Missing Fields
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本文系统来源：http://www.cnblogs.com/huey/p/6120495.html
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