.NET DataTable skips rows on Load(DataReader)

Question

I'm trying to populate a DataTable, to build a LocalReport, using the following:

MySqlCommand cmd = new MySqlCommand();
cmd.Connection = new MySqlConnection(Properties.Settings.Default.dbConnectionString);
cmd.CommandType = CommandType.Text;
cmd.CommandText = "SELECT ... LEFT JOIN ... WHERE ..."; /* query snipped */

// prepare data
dataTable.Clear();
cn.Open();
// fill datatable
dt.Load(cmd.ExecuteReader());
// fill report
rds = new ReportDataSource("InvoicesDataSet_InvoiceTable",dt);
reportViewerLocal.LocalReport.DataSources.Clear();
reportViewerLocal.LocalReport.DataSources.Add(rds);

At one point I noticed that the report was incomplete and it was missing one record. I've changed a few conditions so that the query would return exactly two rows and... surprise: The report shows only one row instead of two. I've tried to debug it to find where the problem is and I got stuck at

 dt.Load(cmd.ExecuteReader());

When I've noticed that the DataReader contains two records but the DataTable contains only one. By accident, I've added an ORDER BY clause to the query and noticed that this time the report showed correctly.

Apparently, the DataReader contains two rows but the DataTable only reads both of them if the SQL query string contains an ORDER BY (otherwise it only reads the last one). Can anyone explain why this is happening and how it can be fixed?

Edit: When I first posted the question, I said it was skipping the first row; later I realized that it actually only read the last row and I've edited the text accordingly (at that time all the records were grouped in two rows and it appeared to skip the first when it actually only showed the last). This may be caused by the fact that it didn't have a unique identifier by which to distinguish between the rows returned by MySQL so adding the ORDER BY statement caused it to create a unique identifier for each row.
This is just a theory and I have nothing to support it, but all my tests seem to lead to the same result.

Answer 1

I had same issue. I took hint from your blog and put up the ORDER BY clause in the query so that they could form together the unique key for all the records returned by query. It solved the problem. Kinda weird.

Answer 2

the question is several years old, but I hadn't found a decent answer, other than the above mentioned workaround.

After fiddling around quite a bit I found that the DataTable.Load method expects a primary key column in the underlying data. If you read the documentation carefully, this becomes obvious, although it is not stated very explicitly.

If you have a column named "id" it seems to use that (which fixed it for me). Otherwise it just seems to use the first column, whether it is unique or not, and overwrites rows with the same value in that column as they are being read. If you don't have a column named "id" and your first column isn't unique, I'd suggest to try to explicitly set the primary key column(s) of the datatable before loading the datareader.

Answer 3

Just in case anyone is having a similar problem as canceriens, I was using If DataReader.Read ... instead of If DataReader.HasRows to check existence before calling dt.load(DataReader) Doh!

Answer 4

Had the same issue. It is because the primary key on all the rows is the same. It's probably what's being used to key the results, and therefore it's just overwriting the same row over and over again.

Datatables.Load points to the fill method to understand how it works. This page states that it is primary key aware. Since primary keys can only occur once and are used as the keys for the row ...

"The Fill operation then adds the rows to destination DataTable objects in the DataSet, creating the DataTable objects if they do not already exist. When creating DataTable objects, the Fill operation normally creates only column name metadata. However, if the MissingSchemaAction property is set to AddWithKey, appropriate primary keys and constraints are also created." (http://msdn.microsoft.com/en-us/library/zxkb3c3d.aspx)

Answer 5

Came across this problem today.

Nothing in this thread fixed it unfortunately, but then I wrapped my SQL query in another SELECT statement and it work!

Eg:

SELECT * FROM (
    SELECT ..... < YOUR NORMAL SQL STATEMENT HERE />
) allrecords

Strange....

Answer 6

Don't use

dr.Read()

Because It moves the pointer to the next row. Remove this line hope it will work.

Answer 7

Can you grab the actual query that is running from SQL profiler and try running it? It may not be what you expected.

Do you get the same result when using a SqlDataAdapter.Fill(dataTable)?

Have you tried different command behaviors on the reader? MSDN Docs

Answer 8

I know this is an old question, but for me the think that worked whilst querying an access database and noticing it was missing 1 row from query, was to change the following:-

    if(dataset.read())  - Misses a row.

    if(dataset.hasrows) - Missing row appears.

Answer 9

Not sure why you're missing the row in the datatable, is it possible you need to close the reader? In any case, here is how I normally load reports and it works every time...

        Dim deals As New DealsProvider()
        Dim adapter As New ReportingDataTableAdapters.ReportDealsAdapter
        Dim report As ReportingData.ReportDealsDataTable = deals.GetActiveDealsReport()
        rptReports.LocalReport.DataSources.Add(New ReportDataSource("ActiveDeals_Data", report))

Curious to see if it still happens.

Answer 10

In my case neither ORDER BY, nor dt.AcceptChanges() is working. I dont know why is that problem for. I am having 50 records in database but it only shows 49 in the datatable. skipping first row, and if there is only one record in datareader it shows nothing at all.

what a bizzareeee.....

Answer 11

Have you tried calling dt.AcceptChanges() after the dt.Load(cmd.ExecuteReader()) call to see if that helps?

Answer 12

I know this is an old question, but I was experiencing the same problem and none of the workarounds mentioned here did help.

In my case, using an alias on the colum that is used as the PrimaryKey solved the issue.

So, instead of

SELECT a
     , b
FROM table

I used

SELECT a as gurgleurp
     , b
FROM table

and it worked.

Answer 13

I had the same problem.. do not used dataReader.Read() at all.. it will takes the pointer to the next row. Instead use directly datatable.load(dataReader).