mongodb 怎么 对类型为list的字段的值进行汇总统计

Question

mongodb中有一个字段为list类型如下，tags想要对tags内的'a、b、c、d'其进行计数统计{'_id':ObjectId('594e473dd746002ad0464b36'),'tags':['a','b','d']}{'_id':ObjectId('59637962d7460028c05590ef'),'tags':['a','c','d']}{'_id':ObjectId('59637962d7460028c0558ff6'),'tags':['c','d']}统计tags内的'a','b','c','d'每个都出现了几次。期待的结果[{'name':'a','count':2},{'name':'b','count':1},{'name':'c','count':2},{'name':'d','count':3}]该怎么写这个查询语句呢？

Answer 1

数据源
db.getCollection('test').insert([
{
'tags':['a','b','d']
},
{
'tags':['a','d']
},
{
'tags':['b','d']
}
])
SQL
db.getCollection('test').aggregate([
{
$unwind:'$tags'
},{
$group:{
_id:'$tags',
count:{$sum:1}
}
}
])
结果
/*1*/
{
"_id":"d",
"count":3.0
}
/*2*/
{
"_id":"b",
"count":2.0
}
/*3*/
{
"_id":"a",
"count":2.0
}
如果你要将查询结果的_id字段名改为name,那么就要再加个$project
db.getCollection('test').aggregate([
{
$unwind:'$tags'
},{
$group:{
_id:'$tags',
count:{$sum:1}
}
},{
$project:{
_id:0,
name:'$_id',
count:'$count'
}
}
])
                            

Answer 2

db.collection.aggregate([
{$unwind:"$tags"},
{$group:{_id:"$tags",count:{$sum:1}}},
{$project:{name:"$_id",count:"$count",_id:0}}
]);
$unwind,$group,$project都是很常用的运算符，先查下文档看咯，不懂再说。