'''
左边是mongodb查询语句，右边是sql语句。对照着用，挺方便。
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 >=18and age <=30// $lt(<) $lte(<=) $gt(>) $gte(>=)
db.users.find({"username": {"$ne":"joe"}}) select * from users where username <>"joe"
db.users.find({"ticket_no": {"$in": [725,542,390]}}) select *from users where ticket_noin(725,542,390)
db.users.find({"ticket_no": {"$nin": [725,542,390]}})select * from users where ticket_no notin(725,542,390)
db.users.find({"$or": [{"ticket_no":725}, {"winner":true}]})select * form users where ticket_no =725or winner =true
db.users.find({"id_num": {"$mod": [5,1]}}) select *from userswhere(id_num mod5)=1
db.users.find({"$not": {"age":27}}) select *from users wherenot(age =27)
db.users.find({"username": {"$in": [null],"$exists":true}})select * from users where username isnull// 如果直接通过find({"username" : null})进行查询，那么连带"没有username"的纪录一并筛选出来
db.users.find({"name": /joey?/i})// 正则查询，value是符合PCRE的表达式
db.food.find({fruit : {$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 -> $where
db.foo.find({"$where":"function() { return this.x + this.y == 10; }"})// $where可以支持javascript函数作为查询条件
db.foo.find().sort({"x":1}).limit(1).skip(10);// 返回第(10, 11]条，按"x"进行排序; 三个limit的顺序是任意的，应该尽量避免skip中使用large-number
'''
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