db.users.find()                => select * from users
db.users.find({"age" : 27})    => select * from users where age = 27
db.users.find({"username" : "joe", "age" : 27})           => select * from users where "username" = "joe" and age = 27
db.users.find({}, {"username" : 1, "email" : 1})          => select username, email from users
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
db.users.find({"age" : {"$gte" : 18, "$lte" : 30}})       => select * from users where age >=18 and age =< 30 // $lt(<) $lte(=<) $gt(>) $gte(>=)
db.users.find({"username" : {"$ne" : "joe"}})             => select * from users where username <> "joe"
db.users.find({"ticket_no" : {"$in" : [725, 542, 390]}})  => select * from users where ticket_no in (725, 542, 390)
db.users.find({"ticket_no" : {"$nin" : [725, 542, 390]}}) => select * from users where ticket_no not in (725, 542, 390)
db.users.find({"$or" : [{"ticket_no" : 725}, {"winner" : true}]})      => select * form users where ticket_no = 725 or winner = true
db.users.find({"id_num" : {"$mod" : [5, 1]}})             => select * from users where (id_num mod 5) = 1
db.users.find({"$not": {"age" : 27}})                     => select * from users where not (age = 27)
db.users.find({"username" : {"$in" : [null], "$exists" : true}})                               => select * from users where username is null // 如果直接通过find({"username" : null})进行查询，那么连带"没有username"的纪录一并筛选出来
db.users.find({"name" : /joey?/i})                        => //  正则查询，value是符合PCRE的表达式
db.food.find({fruit : {$all : ["apple", "banana"]}})      => //  对数组的查询, 字段fruit中，既包含"apple",又包含"banana"的纪录
db.food.find({"fruit.2" : "peach"})                       => //  对数组的查询, 字段fruit中，第3个(从0开始)元素是peach的纪录
db.food.find({"fruit" : {"$size" : 3}})                   => //  对数组的查询, 查询数组元素个数是3的记录，$size前面无法和其他的操作符复合使用
db.users.findOne(criteria, {"comments" : {"$slice" : 10}})=> //  对数组的查询，只返回数组comments中的前十条，还可以{"$slice" : -10}， {"$slice" : [23, 10]}; 分别返回最后10条，和中间10条
db.people.find({"name.first" : "Joe", "name.last" : "Schmoe"})                                 => // 嵌套查询
db.blog.find({"comments" : {"$elemMatch" : {"author" : "joe", "score" : {"$gte" : 5}}}})       => //  嵌套查询，仅当嵌套的元素是数组时使用,
db.foo.find({"$where" : "this.x + this.y == 10"})         => //  复杂的查询，$where当然是非常方便的，但效率低下。对于复杂查询，考虑的顺序应当是 正则 -> MapReduce -> $where