MongoDB Learning (iii)--the use of function tips

$sum function under $group

Returns a sum of numerical values. Ignores Non-numeric values

can only sum numbers, non-numbers do not work

Query for a period of time, the comparison is actually the time of the string

Db. Collectionaaa.find ({"Createtime": {"$gte": "2018-08-15t00:00:00.000z", "$lte": "2018-08-16t23:59:59.000z")})

The properties of the MONGO query can be renamed by $project

Structure for object-nested objects

    {      name:{        fruit:{          name:‘apple‘              }      }    }

If you want to remove the outside of the name hierarchy, directly get inside the fruit content, you can use the $group method fruit grouping, the classification of the property name into _id, if you want to use the original property name, you can rename by $project

MongoDB Learning (iii)--the use of function tips