The importance ( presence + consequence ) of null in mongodb

MongoDBAndNull.js

//Consider this as base collection. It is important to know how the MongoDB interprets 'null'.

> db.items.find();                       
{ "_id" : ObjectId("4ec60c984f5c08053610772f"), "attr1" : "a" }
{ "_id" : ObjectId("4ec60cb14f5c080536107730"), "attr1" : "b" }
{ "_id" : ObjectId("4ec60cb44f5c080536107731"), "attr1" : "c" }
{ "_id" : ObjectId("4ec60d0c4f5c080536107732"), "attr2" : "d" }
{ "_id" : ObjectId("4ec60d444f5c080536107733"), "attr2" : "d", "attr1" : null }
{ "_id" : ObjectId("4ec60fc34f5c080536107734"), "attr1" : "" }

// Do a query, call it query A
> db.items.find({attr1:null});           
{ "_id" : ObjectId("4ec60d0c4f5c080536107732"), "attr2" : "d" }
{ "_id" : ObjectId("4ec60d444f5c080536107733"), "attr2" : "d", "attr1" : null } 

// Do an another query, call it query B
> db.items.find({attr1:{$type:10}});
{ "_id" : ObjectId("4ec60d444f5c080536107733"), "attr2" : "d", "attr1" : null }

// Do an another query, call it query C 
> db.items.find({attr1:{$exists:false}});
{ "_id" : ObjectId("4ec60d0c4f5c080536107732"), "attr2" : "d" }


As you can see the result of query B and C combined together will be the result of query A.

In another words  B (union) C = A . Also  B (intersection) C = 0.