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# 步骤 4：查询分类账中的表
<a name="getting-started.java.step-4"></a>

**重要**  
终止支持通知：现有客户将能够使用 Amazon QLDB，直到 2025 年 7 月 31 日终止支持。有关更多详细信息，请参阅[将亚马逊 QLDB 账本迁移到亚马逊 Aurora PostgreSQL](https://aws.amazon.com/blogs/database/migrate-an-amazon-qldb-ledger-to-amazon-aurora-postgresql/)。

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](ql-reference.md)作为其查询语言，使用 [Amazon Ion](ion.md)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。表扫描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](working.optimize.md)。

**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 [第 5 步：修改分类账中的文档](getting-started.java.step-5.md)。