java-mongodb分组查询

•$project -  可以重构数据
•$match - 可以实现类似query的功能
•$limit - 限制返回个数
•$skip - 同上
•$unwind - 可以将一个包含数组的文档切分成多个, 比如你的文档有数组字段 A, A中有10个元素, 那么 经过 $unwind处理后会产生10个文档，这些文档只有 字段 A不同
•$group - 统计操作， 还提供了一系列子命令
–$avg, $sum …
•$sort  - 排序

接下来要实现4个功能： 
task 1：统计上海学生平均年龄 

从这个需求来讲，要实现功能要有几个步骤: 1. 找出上海的学生. 2. 统计平均年龄 (当然也可以先算出所有省份的平均值再找出上海的)。

select province, avg(age)
from student
where province = '上海'
group by province

Java代码：

/*创建 $match, 作用相当于query*/
DBObject match = new BasicDBObject("$match", new BasicDBObject("province", "上海"));

/* Group操作*/
DBObject groupFields = new BasicDBObject("_id", "$province");
groupFields.put("AvgAge", new BasicDBObject("$avg", "$age"));
DBObject group = new BasicDBObject("$group", groupFields);

/* 查看Group结果 */
AggregationOutput output = connection.aggregate(match, group); // 执行 aggregation命令
System.out.println(output.getCommandResult());

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输出结果：

"result" : [
{ "_id" : "上海" , "AvgAge" : 32.09375}
] ,
"ok" : 1.0

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task2：统计每个省各科平均成绩

首先更具数据库文档结构，subjects是数组形式，需要先分组，然后再进行统计

主要处理步骤如下：

1.先用$unwind 拆数组 2. 按照 province, subject 分租并求各科目平均分

/* 创建 $unwind 操作, 用于切分数组*/
DBObject unwind = new BasicDBObject("$unwind", "$subjects");

/* Group操作*/
DBObject groupFields = new BasicDBObject("_id", new BasicDBObject("subjname", "$subjects.name").append("province", "$province"));
groupFields.put("AvgScore", new BasicDBObject("$avg", "$subjects.scores"));
DBObject group = new BasicDBObject("$group", groupFields);

/* 查看Group结果 */
AggregationOutput output = connection.aggregate(unwind, group);  // 执行 aggregation命令
System.out.println(output.getCommandResult());

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输出结果：

"result" : [
{ "_id" : { "subjname" : "英语" , "province" : "海南"} , "AvgScore" : 58.1} ,
{ "_id" : { "subjname" : "数学" , "province" : "海南"} , "AvgScore" : 60.485} ,
{ "_id" : { "subjname" : "语文" , "province" : "江西"} , "AvgScore" : 55.538} ,
{ "_id" : { "subjname" : "英语" , "province" : "上海"} , "AvgScore" : 57.65625} ,
{ "_id" : { "subjname" : "数学" , "province" : "广东"} , "AvgScore" : 56.690} ,
{ "_id" : { "subjname" : "数学" , "province" : "上海"} , "AvgScore" : 55.671875} ,
{ "_id" : { "subjname" : "语文" , "province" : "上海"} , "AvgScore" : 56.734375} ,
{ "_id" : { "subjname" : "英语" , "province" : "云南"} , "AvgScore" : 55.7301 } ,
.
.
.
.
"ok" : 1.0

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task3: 

将同一省份的科目成绩统计到一起( 即，期望 ‘province’:’xxxxx’, avgscores:[ {‘xxx’:xxx}, ….] 这样的形式)

要做的有一件事，在前面的统计结果的基础上，先用 project将平均分和成绩揉到一起，再按省份group，将各科目的平均分push到一块,使用
group 再次分组.

Mongo m = new Mongo("localhost", 27017);
DB db = m.getDB("test");
DBCollection coll = db.getCollection("student");

/* 创建 $unwind 操作, 用于切分数组*/
DBObject unwind = new BasicDBObject("$unwind", "$subjects");

/* Group操作*/
DBObject groupFields = new BasicDBObject("_id", new BasicDBObject("subjname", "$subjects.name").append("province", "$province"));
groupFields.put("AvgScore", new BasicDBObject("$avg", "$subjects.scores"));
DBObject group = new BasicDBObject("$group", groupFields);

/* Reshape Group Result*/
DBObject projectFields = new BasicDBObject();
projectFields.put("province", "$_id.province");
projectFields.put("subjinfo", new BasicDBObject("subjname","$_id.subjname").append("avgscore", "$AvgScore"));
DBObject project = new BasicDBObject("$project", projectFields);

/* 将结果push到一起*/
DBObject groupAgainFields = new BasicDBObject("_id", "$province");
groupAgainFields.put("avginfo", new BasicDBObject("$push", "$subjinfo"));
DBObject reshapeGroup = new BasicDBObject("$group", groupAgainFields);

/* 查看Group结果 */
AggregationOutput output = coll.aggregate(unwind, group, project, reshapeGroup);
System.out.println(output.getCommandResult());

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"result" : [
{ "_id" : "辽宁" , "avginfo" : [ { "subjname" : "数学" , "avgscore" : 56.46666666666667} , { "subjname" : "英语" , "avgscore" : 52.093333333333334} , { "subjname" : "语文" , "avgscore" : 50.53333333333333}]} ,
{ "_id" : "四川" , "avginfo" : [ { "subjname" : "数学" , "avgscore" : 52.72727272727273} , { "subjname" : "英语" , "avgscore" : 55.90909090909091} , { "subjname" : "语文" , "avgscore" : 57.59090909090909}]} ,
{ "_id" : "重庆" , "avginfo" : [ { "subjname" : "语文" , "avgscore" : 56.077922077922075} , { "subjname" : "英语" , "avgscore" : 54.84415584415584} , { "subjname" : "数学" , "avgscore" : 55.33766233766234}]} ,
{ "_id" : "安徽" , "avginfo" : [ { "subjname" : "英语" , "avgscore" : 55.458333333333336} , { "subjname" : "数学" , "avgscore" : 54.47222222222222} , { "subjname" : "语文" , "avgscore" : 52.80555555555556}]}
.
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.
] , "ok" : 1.0}

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task4:

DBCollection collection = MongoUtils.getCollection_Database(
(String) ServletContextUtils.getSession().getAttribute(
"game"), SysConst.TABLE_WAIGUA_RATIO);

// query
DBObject match = new BasicDBObject("$match", queryParam_n);
// 利用$project拼装group需要的数据
DBObject fields = new BasicDBObject("name", 1);
fields.put("count", 1);
DBObject project = new BasicDBObject("$project", fields);
DBObject groupFields = new BasicDBObject("_id", "$name");
groupFields.put("count", new BasicDBObject("$sum", "$count"));
DBObject group = new BasicDBObject("$group", groupFields);// group
DBObject limit = new BasicDBObject("$limit", Integer.parseInt(n));
DBObject sort = new BasicDBObject("$sort", new BasicDBObject(
"count", -1));

AggregationOutput output = collection.aggregate(match, project,
group, sort, limit);
List<String> nameList = new ArrayList<String>();
for (DBObject obj : output.results()) {
BasicDBObject obj2 = (BasicDBObject) obj;
nameList.add(obj2.getString("_id"));
}