查询表主要需要选择一个表,指定一个分区键,然后执行查询;可以选择使用二级索引并通过扫描操作执行更深层次的过滤.
利用GUI控制台,Java或其他选项来执行任务.
使用以前创建的表执行一些简单查询.首先,在 https://console.aws.amazon.com/dynamodb 上打开控制台
从导航窗格中选择表,然后从表格列表中选择回复.然后选择项目标签以查看加载的数据.
选择创建下方的数据过滤链接("扫描:[表格]回复")项目按钮.
在过滤屏幕中,选择"查询"以进行操作.输入相应的分区键值,然后单击开始.
回复表然后返回匹配的项目.
使用Java中的查询方法,用于执行数据检索操作.它需要指定分区键值,sort键是可选的.
首先创建一个描述参数的 querySpec对象来编写Java查询.然后将对象传递给查询方法.我们使用前面示例中的分区键.
您可以查看以下示例 :
import java.util.HashMap; import java.util.Iterator; import com.amazonaws.services.dynamodbv2.AmazonDynamoDBClient; import com.amazonaws.services.dynamodbv2.document.DynamoDB; import com.amazonaws.services.dynamodbv2.document.Item; import com.amazonaws.services.dynamodbv2.document.ItemCollection; import com.amazonaws.services.dynamodbv2.document.QueryOutcome; import com.amazonaws.services.dynamodbv2.document.Table; import com.amazonaws.services.dynamodbv2.document.spec.QuerySpec; import com.amazonaws.services.dynamodbv2.document.utils.NameMap; public class ProductsQuery { public static void main(String[] args) throws Exception { AmazonDynamoDBClient client = new AmazonDynamoDBClient() .withEndpoint("http://localhost:8000"); DynamoDB dynamoDB = new DynamoDB(client); Table table = dynamoDB.getTable("Products"); HashMap<String, String> nameMap = new HashMap<String, String>(); nameMap.put("#ID", "ID"); HashMap<String, Object> valueMap = new HashMap<String, Object>(); valueMap.put(":xxx", 122); QuerySpec querySpec = new QuerySpec() .withKeyConditionExpression("#ID = :xxx") .withNameMap(new NameMap().with("#ID", "ID")) .withValueMap(valueMap); ItemCollection<QueryOutcome> items = null; Iterator<Item> iterator = null; Item item = null; try { System.out.println("Product with the ID 122"); items = table.query(querySpec); iterator = items.iterator(); while (iterator.hasNext()) { item = iterator.next(); System.out.println(item.getNumber("ID") + ": " + item.getString("Nomenclature")); } } catch (Exception e) { System.err.println("Cannot find products with the ID number 122"); System.err.println(e.getMessage()); } } }
请注意,查询使用分区键,但是,二级索引提供了另一个选项查询.它们的灵活性允许查询非关键属性,这个主题将在本教程后面讨论.
扫描方法还通过收集所有表数据来支持检索操作. 可选.withFilterExpression 可防止指定条件之外的项目出现在结果中.
在本教程的后面部分,我们将讨论扫描详情.现在,看看以下示例 :
import java.util.Iterator; import com.amazonaws.services.dynamodbv2.AmazonDynamoDBClient; import com.amazonaws.services.dynamodbv2.document.DynamoDB; import com.amazonaws.services.dynamodbv2.document.Item; import com.amazonaws.services.dynamodbv2.document.ItemCollection; import com.amazonaws.services.dynamodbv2.document.ScanOutcome; import com.amazonaws.services.dynamodbv2.document.Table; import com.amazonaws.services.dynamodbv2.document.spec.ScanSpec; import com.amazonaws.services.dynamodbv2.document.utils.NameMap; import com.amazonaws.services.dynamodbv2.document.utils.ValueMap; public class ProductsScan { public static void main(String[] args) throws Exception { AmazonDynamoDBClient client = new AmazonDynamoDBClient() .withEndpoint("http://localhost:8000"); DynamoDB dynamoDB = new DynamoDB(client); Table table = dynamoDB.getTable("Products"); ScanSpec scanSpec = new ScanSpec() .withProjectionExpression("#ID, Nomenclature , stat.sales") .withFilterExpression("#ID between :start_id and :end_id") .withNameMap(new NameMap().with("#ID", "ID")) .withValueMap(new ValueMap().withNumber(":start_id", 120) .withNumber(":end_id", 129)); try { ItemCollection<ScanOutcome> items = table.scan(scanSpec); Iterator<Item> iter = items.iterator(); while (iter.hasNext()) { Item item = iter.next(); System.out.println(item.toString()); } } catch (Exception e) { System.err.println("Cannot perform a table scan:"); System.err.println(e.getMessage()); } } }