假设我有一系列这样的书:
{author:"john", category:"action", title:"foobar200"},
{author:"peter", category:"scifi" , title:"42test"},
{author:"peter", category:"novel", title:"whatever_t"},
{author:"jane", category:"novel", title:"the return"},
{author:"john", category:"action", title:"extreme test"},
{author:"peter", category:"scifi", title:"such title"},
{author:"jane", category:"action", title:"super book "}
我想做一个类似于以下的查询:
按类别、作者从图书组中选择作者、类别、计数(*)
==> 结果:
john -> action -> 2
john -> novel -> 0
john -> scifi -> 0
jane -> action -> 1
etc...
我最接近的解决方案是:
db.books.aggregate(
{
$match: {category:"action"}
},
{
$group: { _id: '$author', result: { $sum: 1 } }
}
);
==>结果
{ "_id" : "jane", "result" : 1 }
{ "_id" : "john", "result" : 2 }
{ "_id" : "peter", "result" : 0 }
但我无法理解如何使用类别执行第二个“分组依据”。
做到这一点的最佳方法是什么?
谢谢
最佳答案
您可以在 $group
使用的 _id
中包含多个字段以提供多字段分组:
db.books.aggregate([
{$group: {
_id: {category: '$category', author: '$author'},
result: {$sum: 1}
}}
])
结果:
{
"_id" : {
"category" : "action",
"author" : "jane"
},
"result" : 1
},
{
"_id" : {
"category" : "novel",
"author" : "jane"
},
"result" : 1
},
{
"_id" : {
"category" : "novel",
"author" : "peter"
},
"result" : 1
},
{
"_id" : {
"category" : "scifi",
"author" : "peter"
},
"result" : 2
},
{
"_id" : {
"category" : "action",
"author" : "john"
},
"result" : 2
}
关于node.js - mongodb 中的多个组聚合,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/28280804/