第四章 查询语句：MongoDb VS MySql 4.3

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　　? group
　　这个确实挺高级的，查看示例：
　　db.coll.group(
　　{key: { a: , b:  }, true true
　　cond: { active:1 },
　　reduce: function(obj,prev) { prev.csum += obj.c; },
　　initial: { csum: 0 }
　　});
　　API:
　　public DBObject group(DBObject key,
　　DBObject cond,
　　DBObject initial,
　　String reduce)
　　throws MongoExceptionParameters:
　　key - - { a : true }
　　cond - - optional condition on query
　　reduce - javascript reduce function
　　initial - initial value for first match on a key
　　
　　JAVA中应用例子：
　　System.out.println("-----------华丽分隔线---------------");
　　System.out.println("SELECT sum(isGold)
　　goldCount,discipline,gender,event FROM schedule
　　where isGold=1 group by discipline,gender,event");
　　startTime = System.currentTimeMillis();
　　//first group by key
　　BasicDBObject key = new BasicDBObject();
　　key.put("discipline", true);
　　key.put("gender", true);
　　key.put("event", true);
　　
　　//second where cond
　　BasicDBObject cond = new BasicDBObject();
　　cond.put("isGold", 1);
　　
　　//third initial
　　BasicDBObject initial = new BasicDBObject();
　　initial.put("goldCount", 0);
　　
　　//four String reduce
　　String reduce = "function(obj,prev) { prev.goldCount +=
　　obj.isGold; }";
　　
　　query = new BasicDBObject();
　　
　　field = new BasicDBObject();
　　field.put("rsc",true);
　　field.put("startDate",true);
　　field.put("startTime",true);
　　field.put("competitionStatus",true);
　　sort = new BasicDBObject();
　　sort.put("startDate", 1);
　　sort.put("startTime", 1);
　　DBObject obj = coll.group(key, cond, initial, reduce);
　　//cur = coll.find(query,field).sort(sort);
　　//System.out.println("查询获得的长度"+cur.count());
　　System.out.println("查询耗时:"+(System.currentTimeMillis()-startTime)+"毫秒.");
　　Set set = obj.keySet();
　　Iterator it = set.iterator();
　　int i=0;
　　while(it.hasNext()){
　　String str_key = (String) it.next();
　　System.out.println(obj.get(str_key));
　　i++;
　　}
　　System.out.println("i========"+i);
　　
　　运行结果：
　　-----------华丽分隔线---------------
　　SELECT sum(isGold) goldCount,discipline,gender,event FROM schedule where isGold=1 group by discipline
　　查询耗时:63毫秒.
　　{ "discipline" : "AR" , "goldCount" : 4.0}
　　{ "discipline" : "AT" , "goldCount" : 45.0}
　　{ "discipline" : "BB" , "goldCount" : 1.0}
　　{ "discipline" : "BD" , "goldCount" : 5.0}
　　{ "discipline" : "BK" , "goldCount" : 2.0}
　　{ "discipline" : "BV" , "goldCount" : 2.0}
　　{ "discipline" : "BX" , "goldCount" : 11.0}
　　{ "discipline" : "CB" , "goldCount" : 2.0}
　　{ "discipline" : "CF" , "goldCount" : 12.0}
　　{ "discipline" : "CM" , "goldCount" : 2.0}
　　{ "discipline" : "CR" , "goldCount" : 4.0}
　　{ "discipline" : "CS" , "goldCount" : 4.0}
　　{ "discipline" : "CT" , "goldCount" : 8.0}
　　{ "discipline" : "DV" , "goldCount" : 8.0}
　　{ "discipline" : "EQ" , "goldCount" : 6.0}
　　{ "discipline" : "FB" , "goldCount" : 2.0}
　　{ "discipline" : "FE" , "goldCount" : 10.0}
　　{ "discipline" : "GA" , "goldCount" : 14.0}
　　{ "discipline" : "GR" , "goldCount" : 2.0}
　　{ "discipline" : "GT" , "goldCount" : 2.0}
　　{ "discipline" : "HB" , "goldCount" : 2.0}
　　{ "discipline" : "HO" , "goldCount" : 2.0}
　　{ "discipline" : "JU" , "goldCount" : 14.0}
　　{ "discipline" : "RO" , "goldCount" : 14.0}
　　{ "discipline" : "SA" , "goldCount" : 11.0}
　　{ "discipline" : "SH" , "goldCount" : 15.0}
　　{ "discipline" : "SO" , "goldCount" : 1.0}
　　{ "discipline" : "SW" , "goldCount" : 34.0}
　　{ "discipline" : "SY" , "goldCount" : 2.0}
　　{ "discipline" : "TE" , "goldCount" : 4.0}
　　{ "discipline" : "TK" , "goldCount" : 8.0}
　　{ "discipline" : "TR" , "goldCount" : 2.0}
　　{ "discipline" : "TT" , "goldCount" : 4.0}
　　{ "discipline" : "VO" , "goldCount" : 2.0}
　　{ "discipline" : "WP" , "goldCount" : 2.0}
　　{ "discipline" : "WR" , "goldCount" : 18.0}
　　i========36