關於null：若某列值為null，則設置條件null查詢時，會返回該行文檔，但同時還會返回不存在該列的文檔。
> db.tianyc02.find()
{ "_id" : ObjectId("50ea6eba12729d90ce6e3423"), "name" : "xttt", "age" : 111 }
{ "_id" : ObjectId("50ea6eba12729d90ce6e3424"), "name" : "xttt", "age" : 222 }
{ "_id" : ObjectId("50ea6b6f12729d90ce6e341b"), "name" : "xtt", "age" : 11, "sex" : null }
{ "_id" : ObjectId("50ea6b7312729d90ce6e341c"), "name" : "xtt", "age" : 22, "sex" : "m" }
> db.tianyc02.find({sex:null})
{ "_id" : ObjectId("50ea6eba12729d90ce6e3423"), "name" : "xttt", "age" : 111 }
{ "_id" : ObjectId("50ea6eba12729d90ce6e3424"), "name" : "xttt", "age" : 222 }
{ "_id" : ObjectId("50ea6b6f12729d90ce6e341b"), "name" : "xtt", "age" : 11, "sex" : null }
此時，就需要判斷sex列是否存在：
> db.tianyc02.find({sex:{$exists:true}})
{ "_id" : ObjectId("50ea6b6f12729d90ce6e341b"), "name" : "xtt", "age" : 11, "sex" : null }
{ "_id" : ObjectId("50ea6b7312729d90ce6e341c"), "name" : "xtt", "age" : 22, "sex" : "m" }
> db.tianyc02.find({sex:{$in:[null],$exists:true}})
{ "_id" : ObjectId("50ea6b6f12729d90ce6e341b"), "name" : "xtt", "age" : 11, "sex" : null }