"SequoiaDB giant sequoia database" aggregate() overview 3

Example

Suppose the collection contains records in the following format:

{
  no:1000,
  score:80,
  interest:["basketball","football"],
  major:"计算机科学与技术",
  dep:"计算机学院",
  info:
  {
    name:"Tom",
    age:25,
    gender:"男"
  }
}

Select records based on conditions and specify the return field names. The following aggregation operation first uses $match to select records that match the conditions, and then uses $project to return only the specified field names.

db.sample.employee.aggregate( { $match: { $and: [ { no: { $gt: 1002 } },
                                                { no: { $lt: 1015 } },
                                                { dep: "计算机学院" } ] } },
                        { $project: { no: 1, "info.name": 1, major: 1 } } )
{
        "no": 1003,
        "info.name": "Sam",
        "major": "计算机软件与理论"
    }
    {
        "no": 1004,
        "info.name": "Coll",
        "major": "计算机工程"
    }
    {
        "no": 1005,
        "info.name": "Jim",
        "major": "计算机工程"
    }

Select records based on conditions and group records. The following operation first uses $match to select records that match the conditions, then uses $group to group records by field major, and uses $avg to return the average value of the age field of the nested objects in each group.

db.sample.employee.aggregate( { $match: { dep:  "计算机学院" } },
                        { $group: { _id:  "$major", Major: { $first: "$major" }, 
                          avg_age: { $avg: "$info.age" } } } ) 
{
    "Major": "计算机工程",
    "avg_age": 25
    }
    {
    "Major": "计算机科学与技术",
    "avg_age": 22.5
    }
    {
    "Major": "计算机软件与理论",
    "avg_age": 26
    }

Select records according to conditions, group and sort records, and limit the starting position and number of returned records. The following operations first select the records matching the conditions by $match; then use $group to group by major, and use $avg to return the average value of the age field of the nested objects in each group, the output field name is avg_age; finally, use $sort to The avg_age field value (descending order) and the major field value (descending order) sort the result set, use $skip to determine the starting position of the returned records, and use $limit to limit the number of returned records.

db.sample.employee.aggregate( { $match: { interest: { $exists: 1 } } }, 
                            { $group: { _id: "$major", 
                                        avg_age: { $avg: "$info.age" }, 
                                        major: { $first: "$major" } } }, 
                            { $sort: { avg_age: -1, major: -1 } }, 
                            { $skip: 2 }, 
                            { $limit: 3 } )
{
        "avg_age": 25,
        "major": "计算机科学与技术"
    }
    {
        "avg_age": 22,
        "major": "计算机软件与理论"
    }
    {
        "avg_age": 22,
        "major": "物理学"
    }

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