我正在学习MongoDB。上的“$” 使用越来越困惑我有收集如下模式:
{
_id: 1,
"name": "test",
"city": "gr",
"sector": "IT",
"salary":1000
}
我发现下面的输出下面的查询执行:
Query Result
db.user.find({salary:2000}); Works
db.user.find({$salary:2000}); does not work(unknown top level operator: $salary)
db.user.aggregate({$group:{_id:null,avg:{$avg:"$salary"}}}); Works
db.user.aggregate({$group:{_id:null,avg:{$avg:$salary}}}); does not work($salary is not defined)
db.user.aggregate({$group:{_id:null,avg:{$avg:"salary"}}}); gives wrong output.
谁能请解释一下,什么是的语法意义“”和$在mongoDB中。