在MongoDB中仅对“ $ group”操作中的前两个字段进行排序并获取
让我们创建一个包含文档的集合-
> db.demo576.insertOne({id:101,Name:"Chris",Marks:45}){
"acknowledged" : true, "insertedId" : ObjectId("5e916c3b581e9acd78b427fa")
}
> db.demo576.insertOne({id:101,Name:"John",Marks:55}){
"acknowledged" : true, "insertedId" : ObjectId("5e916c43581e9acd78b427fb")
}
> db.demo576.insertOne({id:101,Name:"John",Marks:65}){
"acknowledged" : true, "insertedId" : ObjectId("5e916c47581e9acd78b427fc")
}
> db.demo576.insertOne({id:102,Name:"David",Marks:37}){
"acknowledged" : true, "insertedId" : ObjectId("5e916c55581e9acd78b427fd")
}
> db.demo576.insertOne({id:102,Name:"David",Marks:75}){
"acknowledged" : true, "insertedId" : ObjectId("5e916c5e581e9acd78b427fe")
}在find()方法的帮助下显示集合中的所有文档-
> db.demo576.find();
这将产生以下输出-
{ "_id" : ObjectId("5e916c3b581e9acd78b427fa"), "id" : 101, "Name" : "Chris", "Marks" : 45 }
{ "_id" : ObjectId("5e916c43581e9acd78b427fb"), "id" : 101, "Name" : "John", "Marks" : 55 }
{ "_id" : ObjectId("5e916c47581e9acd78b427fc"), "id" : 101, "Name" : "John", "Marks" : 65 }
{ "_id" : ObjectId("5e916c55581e9acd78b427fd"), "id" : 102, "Name" : "David", "Marks" : 37 }
{ "_id" : ObjectId("5e916c5e581e9acd78b427fe"), "id" : 102, "Name" : "David", "Marks" : 75 }以下是对查询进行排序并仅获取“$group”操作中的前两个字段：
> var query = [
... {
... "$sort": { "Marks": 1 }
... },
... {
... "$group": {
... "_id": "$id",
... "out": { "$first": "$$ROOT" }
... }
... },
... {
... "$project": {
... "_id": "$out._id",
... "id": "$out.id",
... "Name": "$out.Name",
... "MinMarks": "$out.Marks"
... }
... }
... ]
> db.demo576.aggregate(query);这将产生以下输出-
{ "_id" : ObjectId("5e916c3b581e9acd78b427fa"), "id" : 101, "Name" : "Chris", "MinMarks" : 45 }
{ "_id" : ObjectId("5e916c55581e9acd78b427fd"), "id" : 102, "Name" : "David", "MinMarks" : 37 }