Querying Null and Missed Fields
If we observe the way of inserting the documents we may got some
doubts like
- At any time We can add new fields while insertion ,means all the documents of a collection need not to have same no of fields as a result some documents will have some fields and others doesn't.
- We can pass null values to fields.
- So How to find the documents having the specific fields
- So How to find the fields having null values
Querying Existence of Field
MongoDB provides $exists operator to check the existence of a field. we will use this in combination of (at the place of query) find() methods
Usage Syntax
{fieldname : { $exists: true/false } }
Assume these records are there in users collection
{ "_id" : ObjectId("5774c4b4fc520de27ab8745c"), "userid" : "Alex", }
{ "_id" : ObjectId("5774c4b4fc520de27ab8745d"), "userid" : "Andrew", "Age" : 25}
{ "_id" : ObjectId("5774c4b4fc520de27ab8745e"), "userid" : null, "Age" : 44}
Find the users having age field
>db.users.find({Age:{$exists:true}})
OutPut:-
{ "_id" : ObjectId("5774c4b4fc520de27ab8745d"), "userid" :"Andrew", "Age" : 25}
{ "_id" : ObjectId("5774c4b4fc520de27ab8745e"), "userid" : null, "Age" :44}
Querying NULL Values
To Query the null values MongoDB provides the $type operator. MongoDB stores the data in BSON format, In BSON datatypes null is a datatype with 10 type.
So we can query the null values as
{fieldname:{$type:10}}
Find the users having userid as 'null'
>db.users.find({userid:{$type:10}})
OutPut:-
{ "_id" : ObjectId("5774c4b4fc520de27ab8745e"), "userid" :null, "Age" : 44}