1，实现一个

GroupOperation groupOperation = Aggregation.group("stockno").max("volume").as("volume");

        Aggregation agg = Aggregation.newAggregation(StockQuotation.class,groupOperation);         AggregationResults<java.util.Map> results = mongoTemplate.aggregate(agg,StockQuotation.class,java.util.Map.class);         java.util.Map<String, Integer> classMaxScoreMap = new HashMap<String, Integer>();         if(results.getMappedResults() != null && results.getMappedResults().size() != 0){           for(java.util.Map map:results.getMappedResults()){              classMaxScoreMap.put(map.get("_id").toString(), Integer.valueOf(map.get("volume").toString()));              System.out.println("==============" + map.get("_id").toString() + " | " + Integer.valueOf(map.get("volume").toString()));           }         }

 

打印结果：

==============600004 | 6465485

==============600000 | 74449284

因为Aggregation.group()函数，参数不能为空，所以必须要分组，如果查询整个表里面某个列的最大值，这办法不行

2，写个日期的

//stockno,tradingDay是实体类中的字段，maxTradingDay是别名

        GroupOperation groupOperation = Aggregation.group("stockno").max("tradingDay").as("maxTradingDay");         Aggregation agg = Aggregation.newAggregation(StockQuotation.class,groupOperation);         AggregationResults<java.util.Map> results = mongoTemplate.aggregate(agg,StockQuotation.class,java.util.Map.class);         java.util.Map<String, Object> classMaxScoreMap = new HashMap<String, Object>();         if(results.getMappedResults() != null && results.getMappedResults().size() != 0){           for(java.util.Map map:results.getMappedResults()){              classMaxScoreMap.put(map.get("_id").toString(), map.get("maxTradingDay").toString());              System.out.println("==============" + map.get("_id").toString() + " | " + map.get("maxTradingDay").toString());           }         }

打印结果：

==============600004 | Mon May 12 00:00:00 CST 2008

==============600000 | Mon May 12 00:00:00 CST 2008