本文為英文版的機器翻譯版本，如內容有任何歧義或不一致之處，概以英文版為準。

# 步驟 4：查詢分類帳中的資料表
<a name="getting-started.java.step-4"></a>

**重要**  
支援終止通知：現有客戶將可以使用 Amazon QLDB，直到 07/31/2025 的支援結束為止。如需詳細資訊，請參閱[將 Amazon QLDB Ledger 遷移至 Amazon Aurora PostgreSQL](https://aws.amazon.com/blogs/database/migrate-an-amazon-qldb-ledger-to-amazon-aurora-postgresql/)。

在 Amazon QLDB 分類帳中建立資料表並使用資料載入資料表之後，您可以執行查詢來檢閱您剛插入的車輛註冊資料。QLDB 使用 [PartiQL](ql-reference.md) 做為其查詢語言，而 [Amazon Ion](ion.md) 做為其文件導向資料模型。

PartiQL 是一種開放原始碼、與 SQL 相容的查詢語言，已擴充以使用 Ion。使用 PartiQL，您可以使用熟悉的 SQL 運算子插入、查詢和管理資料。Amazon Ion 是 JSON 的超集合。Ion 是一種開放原始碼、以文件為基礎的資料格式，可讓您靈活地儲存和處理結構化、半結構化和巢狀資料。

在此步驟中，您會使用`SELECT`陳述式從`vehicle-registration`分類帳中的資料表讀取資料。

**警告**  
當您在沒有索引查詢的情況下在 QLDB 中執行查詢時，它會叫用完整資料表掃描。PartiQL 支援此類查詢，因為它與 SQL 相容。不過，*請勿*在 QLDB 中執行生產使用案例的資料表掃描。資料表掃描可能會導致大型資料表的效能問題，包括並行衝突和交易逾時。  
若要避免資料表掃描，您必須在索引欄位或文件 ID 上使用*等式*運算子，以`WHERE`述詞子句執行陳述式；例如， `WHERE indexedField = 123`或 `WHERE indexedField IN (456, 789)`。如需詳細資訊，請參閱[最佳化查詢效能](working.optimize.md)。

**查詢資料表**
+ 編譯並執行下列程式 (`FindVehicles.java`)，以查詢在 分類帳中人員下註冊的所有車輛。

------
#### [ 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，此程式會使用 查詢文件的`Person`資料表`GovId LEWISR261LL`，以取得其`id`中繼資料欄位。  
然後，它會使用本文件`id`做為外部索引鍵，透過 查詢`VehicleRegistration`資料表`PrimaryOwner.PersonId`。它也會`VehicleRegistration`與 `VIN` 欄位上的 `Vehicle` 資料表聯結。

若要了解如何修改 `vehicle-registration` 分類帳中資料表中的文件，請參閱 [步驟 5：修改分類帳中的文件](getting-started.java.step-5.md)。