1、db.users.find()   -- db.getCollection('users').find({})  -- select * from users  2、db.users.find({"age" : 27})   -- select * from users where age = 27  3、db.users.find({"username" : "joe", "age" : 27})   -- select * from users where "username" = "joe" and age = 27  4、db.users.find({}, {"username" : 1, "email" : 1})   -- select username, email from users  5、db.users.find({}, {"username" : 1, "_id" : 0})   -- no case    -- 即时加上了列筛选，_id也会返回，必须显式的阻止_id返回6、db.users.find({"age" : {"$gte" : 18, "$lte" : 30}})   -- select * from users where age >=18 and age <= 30   -- $lt(<) $lte(<=) $gt(>) $gte(>=)  7、db.users.find({"username" : {"$ne" : "joe"}})   -- select * from users where username != "joe"  8、db.users.find({"ticket_no" : {"$in" : [725, 542, 390]}})   -- select * from users where ticket_no in (725, 542, 390)  9、db.users.find({"ticket_no" : {"$nin" : [725, 542, 390]}})   -- select * from users where ticket_no not in (725, 542, 390)  10、db.users.find({"$or" : [{"ticket_no" : 725}, {"winner" : true}]})   -- select * form users where ticket_no = 725 or winner = true  11、db.users.find({"username" : {"$in" : [null], "$exists" : true}})   -- select * from users where username is null   -- 如果直接通过find({"username" : null})进行查询，那么连带"没有username"的纪录一并筛选出来 12、db.users.find({"username" : {"$nin" : [null]}})   -- db.users.find({"username" : {"$ne" : null}})   -- select * from users where username is not null