Querying and nulls

The null value in javascript carries different meanings. When a query involves null, this can have multiple interpretations.

Take the following examples:

> db.foo.insert( { x : 1, y : 1 } )
> db.foo.insert( { x : 2, y : "string" } )
> db.foo.insert( { x : 3, y : null } )
> db.foo.insert( { x : 4 } )

// Query #1
> db.foo.find( { "y" : null } ) 
{ "_id" : ObjectId("4dc1975312c677fc83b5629f"), "x" : 3, "y" : null }
{ "_id" : ObjectId("4dc1975a12c677fc83b562a0"), "x" : 4 }

// Query #2
> db.foo.find( { "y" : { $type : 10 } } )
{ "_id" : ObjectId("4dc1975312c677fc83b5629f"), "x" : 3, "y" : null }

// Query #3
> db.foo.find( { "y" : { $exists : false } } )
{ "_id" : ObjectId("4dc1975a12c677fc83b562a0"), "x" : 4 }

To summarize the three queries:

documents where y has the value null or where y does not exist
documents where y has the value null
documents where y does not exist

Querying and nulls

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