MongoDb聚合 [英] MongoDb aggregation

问题描述

如何使用mongdoDb聚合编写Simial查询

How to write simial query using mongdoDb aggregation

select count(*) as ccount from a group by a.someField order by ccount desc limit 1

使用mondoDb中的分组功能，我想按特定键对我的集合进行分组并返回1行，这种情况发生的频率更高

Using group function in mondoDb, I want to group my collection by specific key and return 1 row, which occurs more often

下面的Java代码，返回列表，该列表按字段someField进行分组，并返回元素的集合，其中每个元素都有2个字段，"someField"和"count"(出现次数)，我想避免第二次在客户端进行迭代以找出最大数量

Code below in Java, return list, that was apply group by on field someField and return collection of elements, where every element has 2 fields, "someField" and "count" (number of occurrence) I would like to avoid second iteration on client side to find out maximum of counts

new GroupCommand(myCollection,
                 new BasicDBObject("someField ", true),
                 null,
                 new BasicDBObject("count", 0),
                "function(key,val){ val.count++;}", 
                 null);

推荐答案

MongoDB的group命令无法使用SQL的顺序或限制.当前开发版本(版本2.1)中的聚合框架提供了$ sort和$ limit运算符:

Nothing like SQL's order or limit is available for MongoDB's group command. The aggregation framework in the current development release (version 2.1) provides $sort and $limit operators:

http://www.mongodb.org/display/DOCS/Aggregation+Framework

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