步骤 4：查询分类账中的表

在 Amazon QLDB 分类账中创建表格和加载数据后，您可运行查询以查看刚刚插入的车辆登记数据。QLDB 使用 PartiQL作为其查询语言，使用 Amazon Ion作为其面向文档的数据模型。

PartiQL 是开源、与 SQL 兼容的查询语言，现已扩展为可与 Ion 配合使用。使用 PartiQL，您可使用熟悉的 SQL 运算符插入、查询和管理数据。Amazon Ion 是 JSON 的超集。Ion 是基于文档的开源数据格式，可让您灵活地存储和处理结构化、半结构化和嵌套数据。

在此步骤中，您将使用SELECT 语句从 vehicle-registration分类账中的表中读取数据。

警告

当您在没有索引查找的情况下在 QLDB 中运行查询时，它会调用全表扫描。PartiQL 之所以支持此类查询，是因为其与 SQL 兼容。但是，切勿在 QLDB 中对生产用例运行表扫描。这可能会导致大型表出现性能问题，包括并发冲突与事务超时。

为避免表扫描，必须在索引字段或文档 ID 上使用相等运算符 (WHERE indexedField = 123或WHERE indexedField IN (456, 789)) 运行带有WHERE谓词子句的语句。有关更多信息，请参阅优化查询性能。

查询表格

• 编译并运行以下程序 (FindVehicles.java)，以查询分类账中某人名下注册的所有车辆。

  2.x

  /*
   * Copyright 2019 Amazon.com, Inc. or its affiliates. All Rights Reserved.
   * SPDX-License-Identifier: MIT-0
   *
   * Permission is hereby granted, free of charge, to any person obtaining a copy of this
   * software and associated documentation files (the "Software"), to deal in the Software
   * without restriction, including without limitation the rights to use, copy, modify,
   * merge, publish, distribute, sublicense, and/or sell copies of the Software, and to
   * permit persons to whom the Software is furnished to do so.
   *
   * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
   * INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
   * PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
   * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
   * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
   * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
   */
  
  package software.amazon.qldb.tutorial;
  
  import java.io.IOException;
  
  import org.slf4j.Logger;
  import org.slf4j.LoggerFactory;
  
  import com.amazon.ion.IonValue;
  
  import software.amazon.qldb.Result;
  import software.amazon.qldb.TransactionExecutor;
  import software.amazon.qldb.tutorial.model.Person;
  import software.amazon.qldb.tutorial.model.SampleData;
  
  /**
   * Find all vehicles registered under a person.
   *
   * This code expects that you have AWS credentials setup per:
   * http://docs.aws.amazon.com/java-sdk/latest/developer-guide/setup-credentials.html
   */
  public final class FindVehicles {
      public static final Logger log = LoggerFactory.getLogger(FindVehicles.class);
  
      private FindVehicles() { }
  
      /**
       * Find vehicles registered under a driver using their government ID.
       *
       * @param txn
       *              The {@link TransactionExecutor} for lambda execute.
       * @param govId
       *              The government ID of the owner.
       * @throws IllegalStateException if failed to convert parameters into {@link IonValue}.
       */
      public static void findVehiclesForOwner(final TransactionExecutor txn, final String govId) {
          try {
              final String documentId = Person.getDocumentIdByGovId(txn, govId);
              final String query = "SELECT v FROM Vehicle AS v INNER JOIN VehicleRegistration AS r "
                      + "ON v.VIN = r.VIN WHERE r.Owners.PrimaryOwner.PersonId = ?";
  
              final Result result = txn.execute(query, Constants.MAPPER.writeValueAsIonValue(documentId));
              log.info("List of Vehicles for owner with GovId: {}...", govId);
              ScanTable.printDocuments(result);
          } catch (IOException ioe) {
              throw new IllegalStateException(ioe);
          }
      }
  
      public static void main(final String... args) {
          final Person person = SampleData.PEOPLE.get(0);
          ConnectToLedger.getDriver().execute(txn -> {
              findVehiclesForOwner(txn, person.getGovId());
          });
      }
  }
  

  1.x

  /*
   * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
   * SPDX-License-Identifier: MIT-0
   *
   * Permission is hereby granted, free of charge, to any person obtaining a copy of this
   * software and associated documentation files (the "Software"), to deal in the Software
   * without restriction, including without limitation the rights to use, copy, modify,
   * merge, publish, distribute, sublicense, and/or sell copies of the Software, and to
   * permit persons to whom the Software is furnished to do so.
   *
   * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
   * INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
   * PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
   * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
   * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
   * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
   */
  
  package software.amazon.qldb.tutorial;
  
  import java.io.IOException;
  
  import org.slf4j.Logger;
  import org.slf4j.LoggerFactory;
  
  import com.amazon.ion.IonValue;
  
  import software.amazon.qldb.Result;
  import software.amazon.qldb.TransactionExecutor;
  import software.amazon.qldb.tutorial.model.Person;
  import software.amazon.qldb.tutorial.model.SampleData;
  
  /**
   * Find all vehicles registered under a person.
   *
   * This code expects that you have AWS credentials setup per:
   * http://docs.aws.amazon.com/java-sdk/latest/developer-guide/setup-credentials.html
   */
  public final class FindVehicles {
      public static final Logger log = LoggerFactory.getLogger(FindVehicles.class);
  
      private FindVehicles() { }
  
      /**
       * Find vehicles registered under a driver using their government ID.
       *
       * @param txn
       *              The {@link TransactionExecutor} for lambda execute.
       * @param govId
       *              The government ID of the owner.
       * @throws IllegalStateException if failed to convert parameters into {@link IonValue}.
       */
      public static void findVehiclesForOwner(final TransactionExecutor txn, final String govId) {
          try {
              final String documentId = Person.getDocumentIdByGovId(txn, govId);
              final String query = "SELECT v FROM Vehicle AS v INNER JOIN VehicleRegistration AS r "
                      + "ON v.VIN = r.VIN WHERE r.Owners.PrimaryOwner.PersonId = ?";
  
              final Result result = txn.execute(query, Constants.MAPPER.writeValueAsIonValue(documentId));
              log.info("List of Vehicles for owner with GovId: {}...", govId);
              ScanTable.printDocuments(result);
          } catch (IOException ioe) {
              throw new IllegalStateException(ioe);
          }
      }
  
      public static void main(final String... args) {
          final Person person = SampleData.PEOPLE.get(0);
          ConnectToLedger.getDriver().execute(txn -> {
              findVehiclesForOwner(txn, person.getGovId());
          }, (retryAttempt) -> log.info("Retrying due to OCC conflict..."));
      }
  }

  注意

  首先，该程序在 Person 表格中查询文档GovId LEWISR261LL，以获取其id元数据字段。

  然后，它使用文档id，通过PrimaryOwner.PersonId作为外键查询VehicleRegistration表。它还结合VIN字段上的Vehicle表VehicleRegistration。

要了解如何修改 vehicle-registration 分类账表格中的文档，请参阅 步骤 5：修改分类账中的文档。