关于null：若某列值为null，则设置条件null查询时，会返回该行文档，但同时还会返回不存在该列的文档。
> 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 }
此时，就需要判断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 }