mongodb中的函数都有哪些

{

    "_id" : "A",

    "company_name" : "公司A",

    "search_frequency" : 212

}

{

    "_id" : "B",

    "company_name" : "公司B",

    "search_frequency" : 160

}

{

    "_id" : "C",

    "company_name" : "公司C",

    "search_frequency" : 553

}

db.getCollection('test').aggregate({"$group":{_id: 'max',max_value:{"$max":"$search_frequency"}}});