mongodb 基础（一）创建集合，简单查询、移除集合种文档文档

1、create a collection the name  is users（the users collection contains field :createDate、name、age、grade）

    example:

    db.users.insertMany([{"createDate":"2024-01-10","name":"zhangsan","age":20,"grade":3},{"createDate":"2024-01-10","name":"lisi","age":30,"grade":2},{"createDate":"2024-01-11","name":"wangwu","age":20,"grade":3},{"createDate":"2024-01-11","name":"zhaoliu","age":30,"grade":4},{"createDate":"2024-01-11","name":"qianqi","age":40,"grade":4}]);

2、query users collecton bases on condition 

    1） single condition query 、example:： db.getCollection("users").find({"_id": ObjectId('65f1c96179aca875733ae5f1')});

     2）multiple condition query 、example : A、db.getCollection("users").find({$and:[{"age": 20},{"_id": ObjectId('65f3a66a27290000470074b7')}]})

                                 database screenshot : B、

    3）remove users collection all document: db.getCollection('users').remove({});

    4)  query users collection number bases on :  db.getCollection("users").find({$and:[{"age": 20}]}).count();

    5)  the numbers of grade group  statistics :      

　　                        

        database sreenshot : 

    6)  user collecton of group statistics bases on match 

  

    databases creenshof: