mongodb判断null值

How do I query for fields that have null values?

Fields in a document may store null values, as in a notional collection, test, with the following documents:

{ _id: 1, cancelDate: null }{ _id: 2 }

Different query operators treat null values differently:

• The { cancelDate : null } query matches documents that either contains the cancelDate field whose value is null orthat do not contain the cancelDate field:

  db.test.find( { cancelDate: null } )

  The query returns both documents:

  { "_id" : 1, "cancelDate" : null }{ "_id" : 2 }

• The { cancelDate : { $type: 10 } } query matches documents that contains the cancelDate field whose value is nullonly; i.e. the value of the cancelDate field is of BSON Type Null (i.e. 10) :

  db.test.find( { cancelDate : { $type: 10 } } )

  The query returns only the document that contains the null value:

  { "_id" : 1, "cancelDate" : null }

• The { cancelDate : { $exists: false } } query matches documents that do not contain the cancelDate field:

  db.test.find( { cancelDate : { $exists: false } } )

  The query returns only the document that does not contain the cancelDate field:

  { "_id" : 2 }

See also

 

The reference documentation for the $type and $exists operators.

$type¶

$type¶

Syntax: { field: { $type: <BSON type> } }

$type selects the documents where thevalue of the field is the specified BSON type.

Consider the following example:

db.inventory.find( { price: { $type : 1 } } )

This query will select all documents in the inventory collection where theprice field value is a Double.

If the field holds an array, the $type operator performs the type check against the array elements and not the field.

Consider the following example where the tags field holds an array:

db.inventory.find( { tags: { $type : 4 } } )

This query will select all documents in the inventory collection where thetags array contains an element that is itself an array.

If instead you want to determine whether the tags field is an array type, use the$where operator:

db.inventory.find( { $where : "Array.isArray(this.tags)" } )

See the SERVER-1475 for more information about the array type.

Refer to the following table for the available BSON types and their corresponding numbers.

TypeNumberDouble1String2Object3Array4Binary data5Undefined (deprecated)6Object id7Boolean8Date9Null10Regular Expression11JavaScript13Symbol14JavaScript (with scope)1532-bit integer16Timestamp1764-bit integer18Min key255Max key127

MinKey and MaxKey compare less than and greater than all other possibleBSON element values, respectively, and exist primarily for internal use.

Note

To query if a field value is a MinKey, you must use the$type with -1 as in the following example:

db.collection.find( { field: { $type: -1 } } )

Example

Consider the following example operation sequence that demonstrates both type comparisonand the special MinKey and MaxKey values:

db.test.insert( {x : 3});db.test.insert( {x : 2.9} );db.test.insert( {x : new Date()} );db.test.insert( {x : true } );db.test.insert( {x : MaxKey } )db.test.insert( {x : MinKey } )db.test.find().sort({x:1}){ "_id" : ObjectId("4b04094b7c65b846e2090112"), "x" : { $minKey : 1 } }{ "_id" : ObjectId("4b03155dce8de6586fb002c7"), "x" : 2.9 }{ "_id" : ObjectId("4b03154cce8de6586fb002c6"), "x" : 3 }{ "_id" : ObjectId("4b031566ce8de6586fb002c9"), "x" : true }{ "_id" : ObjectId("4b031563ce8de6586fb002c8"), "x" : "Tue Jul 25 2012 18:42:03 GMT-0500 (EST)" }{ "_id" : ObjectId("4b0409487c65b846e2090111"), "x" : { $maxKey : 1 } }

To query for the minimum value of a shard key of asharded cluster, use the following operation when connected to themongos:

use configdb.chunks.find( { "min.shardKey": { $type: -1 } } )