MongoDB: How to query for records where field is null or not set?

Question

I have an Email document which has a sent_at date field:

{
  \'sent_at\': Date( 1336776254000 )
}

If this E

Answer 1

db.employe.find({ $and:[ {"dept":{ $exists:false }, "empno": { $in:[101,102] } } ] }).count();