Get Distinct Records Returned From Database

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Hi, I have the following codes which will return multiple rows of records. But one of the drawback is that the records returned may be duplicated. For example, if the database return "Hello", it may return two "Hello". I can change the stored procedure adding a "Distinct" at the SELECT statement but is there any way of change it at the C# codes level instead of stored procedure?
Below are my codes:

C#

public CustomDataCollectionDto GetCustomDataCollectionQuestion(CustomDataCollectionDto customDataCollectionDto, long versionId)
        {
            CustomDataCollectionQuestionDtoList questionList = new CustomDataCollectionQuestionDtoList();
            Database db = DatabaseFactory.CreateDatabase();

            DbCommand selectCommand = db.GetStoredProcCommand("dbo.usp_CustomDataCollectionQuestionByVersionId_Select");
            db.AddInParameter(selectCommand, "p_CustomDataCollectionVersionId", DbType.Int64, versionId);
            db.AddInParameter(selectCommand, "p_CultureName", DbType.String, gsfParameterDto.Culture);

            using (IDataReader dataReader = db.ExecuteReader(selectCommand))
            {
                while (dataReader.Read())
                {
                    questionList.DtoList.Add(ParseDto<CustomDataCollectionQuestionDto>(dataReader));
                }

                customDataCollectionDto.CustomDataCollectionQuestionList = questionList;
            }

            return customDataCollectionDto;
        }

Posted 26-May-15 23:57pm

Jamie888

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2 solutions

Solution 2

Use the DataView ToTable function.

C#

var dataTable = new DataTable();
dataTable.Load(dataReader );
var dataView = new DataView(dataTable);
var distinctDataTable  = dataView.ToTable(true, "Column1", "Column2" ...);

Posted 27-May-15 0:39am

Tammam Koujan

Comments

Jamie888 27-May-15 6:49am

Yes sir. I will give it a try on your suggested method. Thank you. :)

Tammam Koujan 27-May-15 7:07am

You are welcome, for more details about ToTable method check the MSDN link:
https://msdn.microsoft.com/en-us/library/wec2b2e6(v=vs.110).aspx

Jamie888 28-May-15 1:38am

Yes sir I will. Thank you again for your effort in helping me.

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Andy Lanng · Accepted Answer · 2015-05-27T00:33:00

You're using DataReader to perform the select. This is great for efficiency as DataReader is a linear, readonly, forward only process. The drawback is that you can't tell if duplicate records exist until you look at the results.

So. Two ways:

One: Compare the given result with what has already been found:
(I have no idea what CustomDataCollectionQuestionDtoList is so I'll guess that it's the auto generated return type of the stored procedure)

C#

using (IDataReader dataReader = db.ExecuteReader(selectCommand))
  {
    while (dataReader.Read())
    {
      var next = ParseDto<customdatacollectionquestiondto>(dataReader);
      if(!questionList.DtoList.Any(item=>item.a==next.a&&item.b==next.b)
        questionList.DtoList.Add(ParseDto<customdatacollectionquestiondto>(dataReader));
    }
 
    customDataCollectionDto.CustomDataCollectionQuestionList = questionList;
  }
</customdatacollectionquestiondto></customdatacollectionquestiondto>

This is pretty good for small result sets but gets to be a real issue for larger sets.

The other method is to use the build in functionality of the completed set.
(again: no idea what CustomDataCollectionQuestionDtoList is so the available methods may differ)

C#

using (IDataReader dataReader = db.ExecuteReader(selectCommand))
  {
    while (dataReader.Read())
    {
      var next = ParseDto<customdatacollectionquestiondto>(dataReader);
      if(!questionList.DtoList.Any(item=>item.a==next.a&&item.b==next.b)
        questionList.DtoList.Add(ParseDto<customdatacollectionquestiondto>(dataReader));
    }
 
    customDataCollectionDto.CustomDataCollectionQuestionList = questionList.Distinct();
  }
</customdatacollectionquestiondto></customdatacollectionquestiondto>

This works great for Linq objects (hopefully CustomDataCollectionQuestionDtoList is at least an IEnumerable) but is most of your results are duplicates you have to be aware that questionList is going to be a lot larger than is needs to be.

Weigh up the options depending on your result set size and the percent of duplicates.

Hope that helps

have fun ^_^