第四章 查询语句：MongoDb VS MySql 4.3

? 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

本文转自jooben 51CTO博客，原文链接:http://blog.51cto.com/jooben/350753