$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. If <boolean> is false, the query returns only 
the documents that do not contain the field.
Examples
Exists and Not Equal ToConsider the following example: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.

Null Values
Given a collection named records with the following documents:

{ 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 }
Consider the output of the following queries:

Query:

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

 { 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 }
Query:

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

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

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

 { a: 3, b: 2 }
{ a: 4 }
{ b: 2 }

而要查看document中哪些field的值为null，则需要用$in 来查询。详情可以参考http://docs.mongodb.org/manual/reference/operator/query/in/ 页面

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