MongoDB查询以_id分组
> db.demo529.insertOne({"Score":10});{
"acknowledged" : true,
"insertedId" : ObjectId("5e8b1d5bef4dcbee04fbbbe4")
}
> db.demo529.insertOne({"Score":20});{
"acknowledged" : true,
"insertedId" : ObjectId("5e8b1d5fef4dcbee04fbbbe5")
}
> db.demo529.insertOne({"Score":10});{
"acknowledged" : true,
"insertedId" : ObjectId("5e8b1d61ef4dcbee04fbbbe6")
}
> db.demo529.insertOne({"Score":10});{
"acknowledged" : true,
"insertedId" : ObjectId("5e8b1d62ef4dcbee04fbbbe7")
}在find()方法的帮助下显示集合中的所有文档-
> db.demo529.find();
这将产生以下输出-
{ "_id" : ObjectId("5e8b1d5bef4dcbee04fbbbe4"), "Score" : 10 }
{ "_id" : ObjectId("5e8b1d5fef4dcbee04fbbbe5"), "Score" : 20 }
{ "_id" : ObjectId("5e8b1d61ef4dcbee04fbbbe6"), "Score" : 10 }
{ "_id" : ObjectId("5e8b1d62ef4dcbee04fbbbe7"), "Score" : 10 }以下是按_id分组的查询-
> db.demo529.aggregate( [
... {
... $group: {
... _id: null,
... count: { $sum: 1 }
... }
... }
... ] )这将产生以下输出-
{ "_id" : null, "count" : 4 }