首先，我們在資料庫中，mongodb的聚合查詢是這樣寫。

db.getCollection('parking_record').aggregate(
        {$match : {"appId" : "2e1800b22ae70600", "leaveTime" : {"$gt" : ISODate("2017-07-12T00:00:00"), "$lt" : ISODate("2017-07-13T00:00:00")}}},
        {$group : {"_id" : "$leaveMethod", "count" : {$sum : 1}}},
        {$sort : {"_id" : 1}}
    )
 

在java類中，應該怎樣呢？這是我寫的其中一個方法。

（首先要匯入mongodb的java驅動包mongo-java-driver-3.2.2.jar）

/**
	 * 根據日期統計離場方式
	 * @param app_id 外掛ID
	 * @param beginDate 開始日期
	 * @param endDate 結束日期
	 * @return {"ManualLeave":2,"AutoLeave":4}
	 * @throws Exception
	 */
	public String aggregateLeaveMethodByDate(String app_id, Date beginDate, Date endDate) throws Exception {
		MongoCollection<Document> collection = PluginMongo.instance().getDatabase().getCollection(MongoCollectionName.PARKING_RECORD);
		Document sub_match = new Document();
		sub_match.put("appId", app_id);
		sub_match.put("leaveTime", new Document("$gt", beginDate).append("$lt", endDate));
		
		Document sub_group = new Document();
		sub_group.put("_id", "$leaveMethod");
		sub_group.put("count", new Document("$sum", 1));
		
		Document match = new Document("$match", sub_match);
		Document group = new Document("$group", sub_group);
		Document sort = new Document("$sort", new Document("_id", 1));
		
		List<Document> aggregateList = new ArrayList<Document>();
		aggregateList.add(match);
		aggregateList.add(group);
		aggregateList.add(sort);
		
		JSONObject ret_obj = new JSONObject();
		AggregateIterable<Document> resultset = collection.aggregate(aggregateList);
		MongoCursor<Document> cursor = resultset.iterator();
		
		try {
			while(cursor.hasNext()) {
				Document item_doc = cursor.next();
				int leaveMethod = item_doc.getInteger("_id", 0);
				int count = item_doc.getInteger("count", 0);
				
				LeaveMethodEnum leaveMethodVal = LeaveMethodEnum.fromType(leaveMethod);
				ret_obj.put(leaveMethodVal.name(), count);
			}
		} finally {
			cursor.close();
		}
		
		return ret_obj.toJSONString();
	}
 

aggregate的相關sql知識可以參考菜鳥教程：http://www.runoob.com/mongodb/mongodb-aggregate.html