DataTable.Load(FbDataReader) does not load everyth

问题:

There is a closely related question here: .NET DataTable skips rows on Load(DataReader)

I have a SQL Query that returns 169 results. The result looks like this:

CustomerID Customer Name TerminalID Creation Date
     1     First Customer   12345   2010-07-07
     1     First Customer   12346   2010-07-07
     1     First Customer   12347   2010-07-07
     2     Second Customer  23456   2011-04-18

This result is correct.

I entered the query in a C# program and execute it like this:

public DataTable getDataTableFromSql(FbCommand command)
{
    // Create a new datatable
    DataTable result = new DataTable();

    // Set up the connection
    using (FbConnection con = new FbConnection(this.connectionString))
    {
        // Open the connection
        con.Open();

        // Set up the select command
        FbCommand sqlCmd = command;
        // Add the connection to it
        sqlCmd.Connection = con;

        try
        {
            // Get the results
            using (FbDataReader sqlReader = sqlCmd.ExecuteReader())
            {
                // Load the results into the table
                result.Load(sqlReader);
            }
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex);
        }
    }

    // Return the table
    return result;
}

This code is tested and it works fine for many different SQL queries. But for the above query the DataTable only contains 39 results and looks like this:

CustomerID Customer Name TerminalID Creation Date
     1     First Customer   12347   2010-07-07
     2     Second Customer  23456   2011-04-18

I fiddled around with the code a bit and here's what I found out so far: The FbDataReader correctly fetches the results from the database. If I just query for the TerminalID I end up with 169 results in the DataTable. If I query for the CustomerID I recieve 39 results.

Conclusion: The line result.Load(sqlReader) groups the result for CustomerID and throws away all other results, no matter if they can be grouped or not.

Why is this happening? How can I load the result of my query into the DataTable without "losing" any rows due to unlogical grouping? And why does the DataTable "group" the result in the first place?

Note: I also tried all three LoadOptions available for DataTables, all with the same outcome: Only 39 results are loaded into the DataTable.

回答1:

Even if i don't know the problem i would suggest to use a DataAdapter instead. Maybe that works:

// Get the results
using(var da = new FbDataAdapter(sqlCmd))
{
    // Load the results into the table
    da.Fill(result);
}

---

Why that happened? I think this answer in another question explains it.

回答2:

The DataTable.Load method expects a primary key column in the underlying data (i.e. from DataReader). Looks like your procedure does not have any primary key column, or if you have one please user order by in the sql statement so that theDataTable will able to accept it as primary.

This is an very old issue with DataTable.Load and not very well documented. In general SQLDataAdapter is good with DataTable.

In your case, I think as soon as Load finds a duplicate it stops loading data. I have not get this documented anywhere but looks like this the issue.

回答3:

The soultion to the problem is as follows:

DataTable.Load() looks for a Primary Key in the results. If the Primary Key it tries to load into the DataTable already is present in that table, it overwrites the row with the new result.

However, using a different alias for the column with the Primary Key does solve the problem, all the results will get loaded into the DataTable, since it seems the information that this column is a Primary Key is overwritten by using an alias.