Question

我正在将parition键作为ID，即VIN（车辆识别号）排序键作为时间戳，我想执行查询并想知道具有最大时间戳的最新记录（仅基于具有最新时间戳的vin的单个记录）

Answer 1

您可以使用Query API，ScanIndexForward为false，Limit等于1，以获得结果。

ScanIndexForward = false - ＆gt;表示按降序排列排序键为了
限制= 1 - ＆gt;意味着只返回一个项目

示例代码： -

var params = {
    TableName : "yourtablename",
    KeyConditionExpression: "#VIN = :vinvalue",
    ExpressionAttributeNames:{
        "#VIN": "VIN"
    },
    ExpressionAttributeValues: {
        ":vinvalue":"somevinvalue"
    },
    ScanIndexForward : false,
    Limit : 1
};

docClient.query(params, function(err, data) {
    if (err) {
        console.error("Unable to query. Error:", JSON.stringify(err, null, 2));
    } else {
        console.log("Query succeeded.");
        data.Items.forEach(function(item) {
            console.log(" -", item.year + ": " + item.title);
        });
    }
});

Answer 2

        public EventLogEntity fetch(String vin) {

                EventLogEntity eventLogEntityResult = null;
                EventLogEntity entity = new EventLogEntity();
                entity.setId(vin);
                DynamoDBQueryExpression<EventLogEntity> queryExpression =
     new DynamoDBQueryExpression<EventLogEntity>().withHashKeyValues(entity); 

//DynamoDBQueryExpression Works only for Partition, Sort and Indexes in Dynamo Db

                queryExpression.setScanIndexForward(false); // Will give you  result in descending order
                queryExpression.withLimit(1);// Will always give you single record
                List<EventLogEntity> result = dynamoDBMapper.queryPage(EventLogEntity.class, queryExpression).getResults();

                if (result != null && !result.isEmpty()) {
                    eventLogEntityResult = result.get(0);
                }
                return eventLogEntityResult;
            }

Dynamo Db与最新数据

2 个答案: