group聚合：
key: 分组字段cond：过滤条件reduce：curr是当前行result是每组的结果集initial : 组变量初始值finalize: 统计一组后的回调函数
用group求每组count：
db.fruit.group({key:{"name": 1}, cond:{"fruitid": {$lt: 10}}, reduce: function(curr, result){ result.cnt += 1; }, initial: {cnt: 1}})
用group求每组sum：
db.fruit.group({key:{"name": 1}, cond:{"fruitid": {$lt: 10}}, reduce: function(curr, result){ result.cnt += curr.weight; }, initial: {cnt: 0}})
用group求每组最大最小值max/min：
db.fruit.group({key:{"name": 1}, cond:{"fruitid": {$lt: 10}}, reduce: function(curr, result){ if(curr.weight> result.cnt) result.cnt = curr.weight; }, initial: {cnt: 0}})
用group求每组平均值avg：
db.fruit.group({key:{"name": 1}, cond:{}, reduce: function(curr, result){ result.cnt += 1; result.sum += curr.weight }, initial: {cnt: 0, sum: 0}, finalize: function(result) { result.avg = result.sum / result.cnt; }})
注意：
Group不支持shard cluster, 无法分布式运算
分布式要使用aggregate(), mapReduce()
转载于:https://www.cnblogs.com/AK47Sonic/p/7401117.html
相关资源：JAVA上百实例源码以及开源项目