MongoDB语法与现有关系型数据库SQL语法比较

　　MongoDB语法 MySql语法

　　db.test.find({'name':'foobar'}) <==> select * from test where name='foobar'

　　db.test.find() <==> select *from test

　　db.test.find({'ID':10}).count() <==> select count(*) from test where ID=10

　　db.test.find().skip(10).limit(20) <==> select * from test limit 10,20

　　db.test.find({'ID':{$in:[25，35，45]}) <==> select * from test where ID in (25,35,45)

　　db.test.find().sort({'ID':-1}) <==> select * from test order by IDdesc

　　db.test.distinct('name',{'ID':{$lt:20}}) <==> select distinct(name) from testwhere ID<20

　　db.test.group({key:{'name':true},cond:{'name':'foo'},reduce:function(obj,prev){prev.msum+=obj.marks;},initial:{msum:0}}) <==> select name,sum(marks) from testgroup by name

　　db.test.find('this.ID<20',{name:1}) <==> select name from test whereID<20

　　db.test.insert({'name':'foobar','age':25}) <==> insertinto test ('name','age') values('foobar',25)

　　db.test.remove({}) <==> delete * from test

　　db.test.remove({'age':20}) <==> delete test where age=20

　　db.test.remove({'age':{$lt:20}}) <==> delete test where age<20

　　db.test.remove({'age':{$lte:20}}) <==> delete test where age<=20

　　db.test.remove({'age':{$gt:20}}) <==> delete test where age>20

　　db.test.remove({'age':{$gte:20}}) <==> delete test where age>=20

　　db.test.remove({'age':{$ne:20}}) <==> delete test where age!=20

　　db.test.update({'name':'foobar'},{$set:{'age':36}})<==> update test set age=36 where name='foobar'

　　db.test.update({'name':'foobar'},{$inc:{'age':3}})<==> update test set age=age+3 where name='foobar'

　　模糊查询：$regex

　　db.test.find({"name":{$regex:"aaa"}})

　　过滤分组个数

　　db.getCollection('id_mapper').aggregate([{$group:{ _id :"$contract_id",count:{$sum:1}}},{$match:{count:{$gt:1}}}])

　　判断是否为空

　　db.getCollection('id_mapper').find({"sinocardid":{$in:[null]}})