查询表

该DynamoDbTable类的query()方法根据主键值查找项目。@DynamoDbPartitionKey注解和可选@DynamoDbSortKey注解用于定义数据类的主键。

该query()方法需要一个分区键值来查找与提供的值匹配的项目。如果您的表还定义了排序键，则可以在查询中为其添加一个值作为附加比较条件，以微调结果。

除了处理结果外，同步和异步版本query()的工作原理相同。与 API 一样，scanAP query I 为同步调用返回 a，PageIterablePagePublisher为异步调用返回 a。我们PagePublisher之前在扫描部分中讨论了PageIterable和的用法。

Query方法示例

下面的query()方法代码示例使用该MovieActor类。数据类定义了复合主键，该主键由分区键的movie属性和排序键的actor属性组成。

该类还表示它使用名为的全局二级索引acting_award_year。索引的复合主键由分区键的actingaward属性和排序键actingyear的属性组成。在本主题的后面，当我们演示如何创建和使用索引时，我们将参考acting_award_year索引。

MovieActor 类

package org.example.tests.model;

import software.amazon.awssdk.enhanced.dynamodb.mapper.annotations.DynamoDbAttribute;
import software.amazon.awssdk.enhanced.dynamodb.mapper.annotations.DynamoDbBean;
import software.amazon.awssdk.enhanced.dynamodb.mapper.annotations.DynamoDbPartitionKey;
import software.amazon.awssdk.enhanced.dynamodb.mapper.annotations.DynamoDbSecondaryPartitionKey;
import software.amazon.awssdk.enhanced.dynamodb.mapper.annotations.DynamoDbSecondarySortKey;
import software.amazon.awssdk.enhanced.dynamodb.mapper.annotations.DynamoDbSortKey;

import java.util.Objects;

@DynamoDbBean
public class MovieActor implements Comparable<MovieActor> {

    private String movieName;
    private String actorName;
    private String actingAward;
    private Integer actingYear;
    private String actingSchoolName;

    @DynamoDbPartitionKey
    @DynamoDbAttribute("movie")
    public String getMovieName() {
        return movieName;
    }

    public void setMovieName(String movieName) {
        this.movieName = movieName;
    }

    @DynamoDbSortKey
    @DynamoDbAttribute("actor")
    public String getActorName() {
        return actorName;
    }

    public void setActorName(String actorName) {
        this.actorName = actorName;
    }

    @DynamoDbSecondaryPartitionKey(indexNames = "acting_award_year")
    @DynamoDbAttribute("actingaward")
    public String getActingAward() {
        return actingAward;
    }

    public void setActingAward(String actingAward) {
        this.actingAward = actingAward;
    }

    @DynamoDbSecondarySortKey(indexNames = {"acting_award_year", "movie_year"})
    @DynamoDbAttribute("actingyear")
    public Integer getActingYear() {
        return actingYear;
    }

    public void setActingYear(Integer actingYear) {
        this.actingYear = actingYear;
    }

    @DynamoDbAttribute("actingschoolname")
    public String getActingSchoolName() {
        return actingSchoolName;
    }

    public void setActingSchoolName(String actingSchoolName) {
        this.actingSchoolName = actingSchoolName;
    }

    @Override
    public String toString() {
        final StringBuffer sb = new StringBuffer("MovieActor{");
        sb.append("movieName='").append(movieName).append('\'');
        sb.append(", actorName='").append(actorName).append('\'');
        sb.append(", actingAward='").append(actingAward).append('\'');
        sb.append(", actingYear=").append(actingYear);
        sb.append(", actingSchoolName='").append(actingSchoolName).append('\'');
        sb.append('}');
        return sb.toString();
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        MovieActor that = (MovieActor) o;
        return Objects.equals(movieName, that.movieName) && Objects.equals(actorName, that.actorName) && Objects.equals(actingAward, that.actingAward) && Objects.equals(actingYear, that.actingYear) && Objects.equals(actingSchoolName, that.actingSchoolName);
    }

    @Override
    public int hashCode() {
        return Objects.hash(movieName, actorName, actingAward, actingYear, actingSchoolName);
    }

    @Override
    public int compareTo(MovieActor o) {
        if (this.movieName.compareTo(o.movieName) != 0){
            return this.movieName.compareTo(o.movieName);
        } else {
            return this.actorName.compareTo(o.actorName);
        }
    }
}

后面的代码示例对以下项目进行查询。

MovieActor表格中的项目

MovieActor{movieName='movie01', actorName='actor0', actingAward='actingaward0', actingYear=2001, actingSchoolName='null'}
MovieActor{movieName='movie01', actorName='actor1', actingAward='actingaward1', actingYear=2001, actingSchoolName='actingschool1'}
MovieActor{movieName='movie01', actorName='actor2', actingAward='actingaward2', actingYear=2001, actingSchoolName='actingschool2'}
MovieActor{movieName='movie01', actorName='actor3', actingAward='actingaward3', actingYear=2001, actingSchoolName='null'}
MovieActor{movieName='movie01', actorName='actor4', actingAward='actingaward4', actingYear=2001, actingSchoolName='actingschool4'}
MovieActor{movieName='movie02', actorName='actor0', actingAward='actingaward0', actingYear=2002, actingSchoolName='null'}
MovieActor{movieName='movie02', actorName='actor1', actingAward='actingaward1', actingYear=2002, actingSchoolName='actingschool1'}
MovieActor{movieName='movie02', actorName='actor2', actingAward='actingaward2', actingYear=2002, actingSchoolName='actingschool2'}
MovieActor{movieName='movie02', actorName='actor3', actingAward='actingaward3', actingYear=2002, actingSchoolName='null'}
MovieActor{movieName='movie02', actorName='actor4', actingAward='actingaward4', actingYear=2002, actingSchoolName='actingschool4'}
MovieActor{movieName='movie03', actorName='actor0', actingAward='actingaward0', actingYear=2003, actingSchoolName='null'}
MovieActor{movieName='movie03', actorName='actor1', actingAward='actingaward1', actingYear=2003, actingSchoolName='actingschool1'}
MovieActor{movieName='movie03', actorName='actor2', actingAward='actingaward2', actingYear=2003, actingSchoolName='actingschool2'}
MovieActor{movieName='movie03', actorName='actor3', actingAward='actingaward3', actingYear=2003, actingSchoolName='null'}
MovieActor{movieName='movie03', actorName='actor4', actingAward='actingaward4', actingYear=2003, actingSchoolName='actingschool4'}

以下代码定义了两个QueryConditional实例。 QueryConditionals使用键值（单独使用分区键或与排序键结合使用），对应于 DynamoDB 服务 API 的键条件表达式。在注释第 1 行之后，该示例定义了与分区值为的项目匹配的keyEqual实例movie01。

此示例还定义了一个筛选表达式，该表达式可过滤掉注释行 2 之后没有actingschoolname开启的任何项目。

在注释第 3 行之后，该示例显示了代码传递给该DynamoDbTable.query()方法的QueryEnhancedRequest实例。此对象结合了 SDK 用来生成对 DynamoDB 服务的请求的密钥条件和过滤器。

    public static void query(DynamoDbTable movieActorTable) {

        // 1. Define a QueryConditional instance to return items matching a partition value.
        QueryConditional keyEqual = QueryConditional.keyEqualTo(b -> b.partitionValue("movie01"));
        // 1a. Define a QueryConditional that adds a sort key criteria to the partition value criteria.
        QueryConditional sortGreaterThanOrEqualTo = QueryConditional.sortGreaterThanOrEqualTo(b -> b.partitionValue("movie01").sortValue("actor2"));
        // 2. Define a filter expression that filters out items whose attribute value is null.
        final Expression filterOutNoActingschoolname = Expression.builder().expression("attribute_exists(actingschoolname)").build();

        // 3. Build the query request.
        QueryEnhancedRequest tableQuery = QueryEnhancedRequest.builder()
                .queryConditional(keyEqual)
                .filterExpression(filterOutNoActingschoolname)
                .build();
        // 4. Perform the query.
        PageIterable<MovieActor> pagedResults = movieActorTable.query(tableQuery);
        logger.info("page count: {}", pagedResults.stream().count()); // Log  number of pages.

        pagedResults.items().stream()
                .sorted()
                .forEach(
                        item -> logger.info(item.toString()) // Log the sorted list of items.
                );

以下是运行该方法的输出。输出显示movieName值为 movie01 的项目，不显示值actingSchoolName等于的项目。null

2023-03-05 13:11:05 [main] INFO  org.example.tests.QueryDemo:46 - page count: 1
2023-03-05 13:11:05 [main] INFO  org.example.tests.QueryDemo:51 - MovieActor{movieName='movie01', actorName='actor1', actingAward='actingaward1', actingYear=2001, actingSchoolName='actingschool1'}
2023-03-05 13:11:05 [main] INFO  org.example.tests.QueryDemo:51 - MovieActor{movieName='movie01', actorName='actor2', actingAward='actingaward2', actingYear=2001, actingSchoolName='actingschool2'}
2023-03-05 13:11:05 [main] INFO  org.example.tests.QueryDemo:51 - MovieActor{movieName='movie01', actorName='actor4', actingAward='actingaward4', actingYear=2001, actingSchoolName='actingschool4'}

在先前显示的注释行 3 之后的以下查询请求变体中，代码将替换为sortGreaterThanOrEqualToQueryConditional在注释行 1a 之后定义的变体。keyEqual QueryConditional以下代码还删除了过滤器表达式。

        QueryEnhancedRequest tableQuery = QueryEnhancedRequest.builder()
                .queryConditional(sortGreaterThanOrEqualTo)

由于此表具有复合主键，因此所有QueryConditional实例都需要分区键值。 QueryConditional以开头的方法sort...表示需要排序键。结果未排序。

以下输出显示了查询的结果。该查询返回movieName值等于 movie01 的项目，并且仅返回actorName值大于或等于 actor2 的项目。由于过滤器已被移除，因此查询返回的项目没有该actingSchoolName属性值。

2023-03-05 13:15:00 [main] INFO  org.example.tests.QueryDemo:46 - page count: 1
2023-03-05 13:15:00 [main] INFO  org.example.tests.QueryDemo:51 - MovieActor{movieName='movie01', actorName='actor2', actingAward='actingaward2', actingYear=2001, actingSchoolName='actingschool2'}
2023-03-05 13:15:00 [main] INFO  org.example.tests.QueryDemo:51 - MovieActor{movieName='movie01', actorName='actor3', actingAward='actingaward3', actingYear=2001, actingSchoolName='null'}
2023-03-05 13:15:00 [main] INFO  org.example.tests.QueryDemo:51 - MovieActor{movieName='movie01', actorName='actor4', actingAward='actingaward4', actingYear=2001, actingSchoolName='actingschool4'}