语句的比较啊!!! 比如sql的分组语句 select classesid, name ,password from Classes group by classesid;我想知道的是在mongodb 中这条语句怎么写. 不是说在mongodb的分组中 key是group by 的字段 BasicDBObject key = new BasicDBObject(); key.put("groupid", true);
//cond是where的条件 可以为空 BasicDBObject cond = new BasicDBObject();
//initial是字段sum的初始值 BasicDBObject initial = new BasicDBObject(); initial.put("fidCount", 0);
比如sql的分组语句 select classesid, name ,password from Classes group by classesid;我想知道的是在mongodb 中这条语句怎么写.
不是说在mongodb的分组中
key是group by 的字段
BasicDBObject key = new BasicDBObject();
key.put("groupid", true);
//cond是where的条件 可以为空
BasicDBObject cond = new BasicDBObject();
//initial是字段sum的初始值
BasicDBObject initial = new BasicDBObject();
initial.put("fidCount", 0);
//reduce是分解归纳求和 fid的sum
String reduce = "function(obj,prev) { prev.fidCount +=obj.fid; }";
DBObject obj = collection.group(key, cond, initial, reduce);