MongoDB聚合以获取标记最少的两个文档
要获得标记的排序列表，请使用$sort。使用$limit：2仅显示两个带有最少标记的此类文档。让我们创建一个包含文档的集合-
> db.demo709.insertOne({Name:"John","Marks":75});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ea839005d33e20ed1097b76")
}
> db.demo709.insertOne({Name:"Chris","Marks":45});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ea839075d33e20ed1097b77")
}
> db.demo709.insertOne({Name:"David","Marks":54});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ea839125d33e20ed1097b78")
}
> db.demo709.insertOne({Name:"Bob","Marks":69});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ea839295d33e20ed1097b79")
}在find()方法的帮助下显示集合中的所有文档-
> db.demo709.find();
这将产生以下输出-
{ "_id" : ObjectId("5ea839005d33e20ed1097b76"), "Name" : "John", "Marks" : 75 }
{ "_id" : ObjectId("5ea839075d33e20ed1097b77"), "Name" : "Chris", "Marks" : 45 }
{ "_id" : ObjectId("5ea839125d33e20ed1097b78"), "Name" : "David", "Marks" : 54 }
{ "_id" : ObjectId("5ea839295d33e20ed1097b79"), "Name" : "Bob", "Marks" : 69 }以下是获取带有最少标记的两个文档的查询-
> db.demo709.aggregate({
... $group: {
... _id: '$Marks',
... ListOfName: { $push: '$Name' }
... }
... }, {
... $sort: {
... '_id': 1
... }
... }, {
... $limit: 2
... });这将产生以下输出-
{ "_id" : 45, "ListOfName" : [ "Chris" ] }
{ "_id" : 54, "ListOfName" : [ "David" ] }