爬虫中mongodb聚合中$group的使用方法是什么？

分组

db.stu.aggregate( {KaTeX parse error: Expected '}', got 'EOF' at end of input: group:{_id:"hometown",count:{KaTeX parse error: Expected 'EOF', got '}' at position 6: sum:1}̲,total_age:{sum:“KaTeX parse error: Expected 'EOF', got '}' at position 5: age"}̲,avg_age:{avg:”$age"}}} )

_id分组的依据

$age 取age对应的值

$sum:1 把每条数据作为1进行统计，统计的是个数

s u m : " sum:"sum:"age" 统计年龄对应的和

$group对应的字典中的键是输出数据的键

不分组

db.stu.aggregate( {KaTeX parse error: Expected '}', got 'EOF' at end of input: …id:null,count:{sum:1}}} )

按照一个字段分组 db.col.aggregate( {KaTeX parse error: Expected '}', got 'EOF' at end of input: group:{_id:"gender",count:{KaTeX parse error: Expected 'EOF', got '}' at position 6: sum:1}̲}} ) 按照多个…group:{_id:{gender:“g e n d e r " , h o m e t o w n : " gender",hometown:"gender",hometown:"hometown”},count:{KaTeX parse error: Expected 'EOF', got '}' at position 6: sum:1}̲}} ) 不分组,…group:{_id:null,count:{$sum:1}}} ) KaTeX parse error: Expected '}', got 'EOF' at end of input: …ggregate( {group:{_id:“KaTeX parse error: Expected '}', got 'EOF' at end of input: gender",name:{push:”$name"}}} )