想要用java做分组查询，貌似2.4版的java驱动包不支持db.collection.aggregate语法，所以被迫改用db.collection.group()方法。
需求：要用Mongo实现该与SQL的相同作用：
select number from user group by number having count(*) = 1
使用aggregate可以这么实现：
db.user.aggregate(
{$group:{_id:"$number",total:{$sum:1}}},
{$match:{total:1}}
);
现在改用group方法：
db.user.group({
key : {number:1},
cond : {count:1},
initial : {count: 0},
reduce : function Reduce(doc, out) {
out.count +=1;
},
finalize : function Finalize(out) {
return out;
}
});
查不出结果，如何解决？
![[Web front-end] Node.js quick start](https://img.php.cn/upload/course/000/000/067/662b5d34ba7c0227.png)
I tried another method, but it still didn’t work
By the way, is there any use for return in finalize?
2.4 was released in 2010. The MongoDB server released at that time is no longer officially supported, so it is necessary to upgrade.
For aggregation, version 3.2 or above is recommended. It is recommended to use https://github.com/T-baby/Mon... to operate and manage mongodb. Please refer to https://t-baby.gitbooks.io/mo... . Get started in ten minutes