如何维护 MongoDB 中数组元素的最高计数？
您可以使用聚合框架。让我们首先创建一个包含文档的集合-
> db.topCountArrayDemo.insertOne(
... {"StudentId":101 , "StudentSubject": ["C", "MongoDB"]}
... );
{
"acknowledged" : true,
"insertedId" : ObjectId("5cc6b3209cb58ca2b005e669")
}
> db.topCountArrayDemo.insertOne(
... {"StudentId":102 , "StudentSubject": ["C", "Java"]}
... );
{
"acknowledged" : true,
"insertedId" : ObjectId("5cc6b3219cb58ca2b005e66a")
}
> db.topCountArrayDemo.insertOne(
... {"StudentId":103 , "StudentSubject": ["C", "MongoDB"]}
... );
{
"acknowledged" : true,
"insertedId" : ObjectId("5cc6b3229cb58ca2b005e66b")
}以下是在find()方法的帮助下显示集合中所有文档的查询-
> db.topCountArrayDemo.find().pretty();
这将产生以下输出-
{
"_id" : ObjectId("5cc6b3209cb58ca2b005e669"),
"StudentId" : 101,
"StudentSubject" : [
"C",
"MongoDB"
]
}
{
"_id" : ObjectId("5cc6b3219cb58ca2b005e66a"),
"StudentId" : 102,
"StudentSubject" : [
"C",
"Java"
]
}
{
"_id" : ObjectId("5cc6b3229cb58ca2b005e66b"),
"StudentId" : 103,
"StudentSubject" : [
"C",
"MongoDB"
]
}以下是维护MongoDB中数组元素的最高计数的查询-
> db.topCountArrayDemo.aggregate(
... [
... {
... $unwind: "$StudentSubject"
... },
... {
... $group: {
... _id: "$StudentSubject",
... Frequency: {$sum: 1}
... }
... },
... {
... $sort: {Frequency:-1}
... },
... {
... $limit: 2
... }
... ]
... ).pretty();这将产生以下输出-
{ "_id" : "C", "Frequency" : 3 }
{ "_id" : "MongoDB", "Frequency" : 2 }