Querying Tables Using the AWS SDK for .NET Low-Level API

The Query method supports several optional parameters. For example, you can optionally filter the query result in the preceding query to return replies in the past two weeks by specifying a condition. The condition is called range condition because Amazon DynamoDB evaluates the query condition that you specify against the range attribute of the primary key. You can specify other optional parameters to retrieve only a specific list of attributes from items in the query result. For more information about the parameters and the API, see Query.

The following C# code snippet retrieves forum thread replies posted in the past 15 days. The snippet specifies the following optional parameters:

DateTime twoWeeksAgoDate = DateTime.UtcNow - TimeSpan.FromDays(15);
string twoWeeksAgoString = twoWeeksAgoDate.ToString(AWSSDKUtils.ISO8601DateFormat);

var request = new QueryRequest
{
  TableName = "Reply",
  KeyConditions = new Dictionary<string,Condition>()
  {
    {
      "Id",
      new Condition()
      {
        ComparisonOperator = "EQ",
        AttributeValueList = new List<AttributeValue>()
        {
          new AttributeValue { S = "Amazon DynamoDB#DynamoDB Thread 2" }
        }
      }
    }
  },
  {
    {
      "ReplyDateTime",
      new Condition()
      {
        ComparisonOperator = "GT",
        AttributeValueList = new List<AttributeValue>()
        {
          new AttributeValue { S = twoWeeksAgoString }
        }
      }
    }
  },
  AttributesToGet = new List<string> { "Subject", "ReplyDateTime", "PostedBy" },
  ConsistentRead = true
};

var response = client.Query(request);
var result = response.QueryResult;

foreach (Dictionary<string, AttributeValue> item 
  in response.QueryResult.Items)
{
  // Process the result.
  PrintItem(item);
}

You can also optionally limit the page size, or the number of items per page, by adding the optional Limit parameter. Each time you execute the Query method, you get one page of results that has the specified number of items. To fetch the next page, you execute the Query method again by providing the primary key value of the last item in the previous page so that the method can return the next set of items. You provide this information in the request by setting the ExclusiveStartKey property. Initially, this property can be null. To retrieve subsequent pages, you must update this property value to the primary key of the last item in the preceding page.

The following C# code snippet queries the Reply table. In the request, it specifies the Limit and ExclusiveStartKey optional parameters. The do/while loop continues to scan one page at time until the LastEvaluatedKey returns a null value.

Dictionary<string,AttributeValue> lastKeyEvaluated = null;
do
{
  var request = new QueryRequest
  {
    TableName = "Reply",
    KeyConditions = new Dictionary<string,Condition>()
    {
      {
        "Id",
        new Condition()
        {
          ComparisonOperator = "EQ",
          AttributeValueList = new List<AttributeValue>()
          {
            new AttributeValue { S = "Amazon DynamoDB#DynamoDB Thread 2" }
          }
        }
      }
    },
    // Optional parameters.
    Limit = 10,
    ExclusiveStartKey = lastKeyEvaluated
  };

  var response = client.Query(request);
  // Process the query result.
  foreach (Dictionary<string, AttributeValue> item in response.QueryResult.Items)
  {
    PrintItem(item);
  }
  lastKeyEvaluated = response.QueryResult.LastEvaluatedKey;

} while (lastKeyEvaluated != null);

The examples in this section use AWS SDK for .NET low-level API. The SDK also supports helper API that further simplify your application development. Additionally, the SDK provides Object Persistence Model API that enable you to map your client-side classes to your Amazon DynamoDB tables and perform query operations on them. For more information, see Using the AWS SDK for .NET.

The following tables store information about a collection of forums. For more information about table schemas, see Example Tables and Data in Amazon DynamoDB.

Forum ( Name, ... ) 
Thread ( ForumName, Subject, Message, LastPostedBy, LastPostDateTime, ...) 
Reply ( Id, ReplyDateTime, Message, PostedBy, ...)

In this C# code example, you execute variations of "Find replies for a thread "DynamoDB Thread 1" in forum "Amazon DynamoDB".

For step-by-step instructions to test the following sample, see Using the AWS SDK for .NET.

using System;
using System.Collections.Generic;
using Amazon.DynamoDBv2;
using Amazon.DynamoDBv2.Model;
using Amazon.Runtime;
using Amazon.Util;

namespace Amazon.DynamoDBv2.Documentation
{
  class Program
  {
    private static AmazonDynamoDBClient client;

    static void Main(string[] args)
    {
      try
      {
        var config = new AmazonDynamoDBConfig();
        config.ServiceURL = System.Configuration.ConfigurationManager.AppSettings["ServiceURL"];
        client = new AmazonDynamoDBClient(config);

        // Query a specific forum and thread.
        string forumName = "Amazon DynamoDB";
        string threadSubject = "DynamoDB Thread 1";

        FindRepliesForAThread(forumName, threadSubject);
        FindRepliesForAThreadSpecifyOptionalLimit(forumName, threadSubject);
        FindRepliesInLast15DaysWithConfig(forumName, threadSubject);
        FindRepliesPostedWithinTimePeriod(forumName, threadSubject);

        Console.WriteLine("Example complete. To continue, press Enter");
        Console.ReadLine();
      }
      catch (AmazonDynamoDBException e) { Console.WriteLine(e.Message); }
      catch (AmazonServiceException e) { Console.WriteLine(e.Message); }
      catch (Exception e) { Console.WriteLine(e.Message); } 
    }

    private static void FindRepliesPostedWithinTimePeriod(string forumName, string threadSubject)
    {
      Console.WriteLine("*** Executing FindRepliesPostedWithinTimePeriod() ***");
      string replyId = forumName + "#" + threadSubject;
      // You must provide date value based on your test data.
      DateTime startDate = DateTime.UtcNow - TimeSpan.FromDays(21);
      string start = startDate.ToString(AWSSDKUtils.ISO8601DateFormat);

      // You provide date value based on your test data.
      DateTime endDate = DateTime.UtcNow - TimeSpan.FromDays(5);
      string end = endDate.ToString(AWSSDKUtils.ISO8601DateFormat);

      var request = new QueryRequest
      {
        TableName = "Reply",
        ReturnConsumedCapacity = "TOTAL",
        KeyConditions = new Dictionary<string, Condition>()
        {
          {
            "Id",
            new Condition()
            {
              ComparisonOperator = "EQ",
              AttributeValueList = new List<AttributeValue>()
              {
                new AttributeValue { S = replyId }
              }
            }
          },
          {
            "ReplyDateTime",
            new Condition
            {
              ComparisonOperator = "BETWEEN",
              AttributeValueList = new List<AttributeValue>()
              {
                new AttributeValue { S = start },
                new AttributeValue { S = end }
              }
            }
          }
        }
      };

      var response = client.Query(request);
      var result = response.QueryResult;

      Console.WriteLine("\nNo. of reads used (by query in FindRepliesPostedWithinTimePeriod) {0}",
                        response.QueryResult.ConsumedCapacity.CapacityUnits);
      foreach (Dictionary<string, AttributeValue> item
        in response.QueryResult.Items)
      {
        PrintItem(item);
      }
      Console.WriteLine("To continue, press Enter");
      Console.ReadLine();
    }

    private static void FindRepliesInLast15DaysWithConfig(string forumName, string threadSubject)
    {
      Console.WriteLine("*** Executing FindRepliesInLast15DaysWithConfig() ***");
      string replyId = forumName + "#" + threadSubject;

      DateTime twoWeeksAgoDate = DateTime.UtcNow - TimeSpan.FromDays(15);
      string twoWeeksAgoString =
        twoWeeksAgoDate.ToString(AWSSDKUtils.ISO8601DateFormat);

      var request = new QueryRequest
      {
        TableName = "Reply",
        ReturnConsumedCapacity = "TOTAL",
        KeyConditions = new Dictionary<string, Condition>()
        {
          {
            "Id",
            new Condition
            {
              ComparisonOperator = "EQ",
              AttributeValueList = new List<AttributeValue>()
              {
                new AttributeValue { S = replyId }
              }
            }
          },
          {
            "ReplyDateTime",
            new Condition
            {
              ComparisonOperator = "GT",
              AttributeValueList = new List<AttributeValue>()
              {
                new AttributeValue { S = twoWeeksAgoString }
              }
            }
          }
        },
        // Optional parameter.
        AttributesToGet = new List<string> { "Id", "ReplyDateTime", "PostedBy" },
        // Optional parameter.
        ConsistentRead = true
      };

      var response = client.Query(request);
      var result = response.QueryResult;

      Console.WriteLine("No. of reads used (by query in FindRepliesInLast15DaysWithConfig) {0}",
                        response.QueryResult.ConsumedCapacity.CapacityUnits);
      foreach (Dictionary<string, AttributeValue> item
        in response.QueryResult.Items)
      {
        PrintItem(item);
      }
      Console.WriteLine("To continue, press Enter");
      Console.ReadLine();
    }

    private static void FindRepliesForAThreadSpecifyOptionalLimit(string forumName, string threadSubject)
    {
      Console.WriteLine("*** Executing FindRepliesForAThreadSpecifyOptionalLimit() ***");
      string replyId = forumName + "#" + threadSubject;

      Dictionary<string, AttributeValue> lastKeyEvaluated = null;
      do
      {
        var request = new QueryRequest
        {
          TableName = "Reply",
          ReturnConsumedCapacity = "TOTAL",
          KeyConditions = new Dictionary<string, Condition>()
          {
            {
              "Id",
              new Condition
              {
                ComparisonOperator = "EQ",
                AttributeValueList = new List<AttributeValue>()
                {
                  new AttributeValue { S = replyId }
                }
              }
            }
          },
          Limit = 2, // The Reply table has only a few sample items. So the page size is smaller.
          ExclusiveStartKey = lastKeyEvaluated
        };

        var response = client.Query(request);

        Console.WriteLine("No. of reads used (by query in FindRepliesForAThreadSpecifyLimit) {0}\n",
                          response.QueryResult.ConsumedCapacity.CapacityUnits);
        foreach (Dictionary<string, AttributeValue> item
          in response.QueryResult.Items)
        {
          PrintItem(item);
        }
        lastKeyEvaluated = response.QueryResult.LastEvaluatedKey;

      } while (lastKeyEvaluated != null);
      Console.WriteLine("To continue, press Enter");


      Console.ReadLine();
    }

    private static void FindRepliesForAThread(string forumName, string threadSubject)
    {
      Console.WriteLine("*** Executing FindRepliesForAThread() ***");
      string replyId = forumName + "#" + threadSubject;

      var request = new QueryRequest
      {
        TableName = "Reply",
        ReturnConsumedCapacity = "TOTAL",
        KeyConditions = new Dictionary<string, Condition>()
        {
          {
            "Id",
            new Condition
            {
              ComparisonOperator = "EQ",
              AttributeValueList = new List<AttributeValue>()
              {
                new AttributeValue { S = replyId }
              }
            }
          }
        }
      };

      var response = client.Query(request);
      var result = response.QueryResult;
      Console.WriteLine("No. of reads used (by query in FindRepliesForAThread) {0}\n",
                        response.QueryResult.ConsumedCapacity.CapacityUnits);
      foreach (Dictionary<string, AttributeValue> item
        in response.QueryResult.Items)
      {
        PrintItem(item);
      }
      Console.WriteLine("To continue, press Enter");
      Console.ReadLine();
    }

    private static void PrintItem(
     Dictionary<string, AttributeValue> attributeList)
    {
      foreach (KeyValuePair<string, AttributeValue> kvp in attributeList)
      {
        string attributeName = kvp.Key;
        AttributeValue value = kvp.Value;

        Console.WriteLine(
            attributeName + " " +
            (value.S == null ? "" : "S=[" + value.S + "]") +
            (value.N == null ? "" : "N=[" + value.N + "]") +
            (value.SS == null ? "" : "SS=[" + string.Join(",", value.SS.ToArray()) + "]") +
            (value.NS == null ? "" : "NS=[" + string.Join(",", value.NS.ToArray()) + "]")
        );
      }
      Console.WriteLine("************************************************");
    }

  }
}