Question

I have a SQL SELECT statement which will not be known until runtime, which could contain JOIN’s and inner selects. I need to determine the names and data types of each of the columns of the returned result of the statment from within C#. I am inclined to do something like:

string orginalSelectStatement = "SELECT * FROM MyTable";

string selectStatement = string.Format("SELECT TOP 0 * FROM ({0}) s", orginalSelectStatement);
SqlConnection connection = new SqlConnection(@"MyConnectionString");
SqlDataAdapter adapter = new SqlDataAdapter(selectStatement, connection);

DataTable table = new DataTable();
adapter.Fill(table);

foreach (DataColumn column in table.Columns)
{
    Console.WriteLine("Name: {0}; Type: {1}", column.ColumnName, column.DataType);
}

Is there a better way to do what I am trying to do? By "better" I mean either a less resource-intensive way of accomplishing the same task or a more sure way of accomplishing the same task (i.e. for all I know the code snippet I just gave will fail in some situations).

SOLUTION:
First of all, my TOP 0 hack is bad, namely for something like this:

SELECT TOP 0 * FROM (SELECT 0 AS A, 1 AS A) S

In other words, in a sub-select, if two things are aliased to the same name, that throws an error. So it is out of the picture. However, for completeness sake, I went ahead and tested it, along with the two proposed solutions: SET FMTONLY ON and GetSchemaTable.

Here are the results (in milliseconds for 1,000 queries, each):

Schema Time: 3130

TOP 0 Time: 2808

FMTONLY ON Time: 2937

My recommendation would be GetSchemaTable since it’s more likely to be future-proofed by a removal of the SET FMTONLY ON as valid SQL and it solves the aliasing problem, even though it is slightly slower. However, if you "know" that duplicate column names will never be an issue, then TOP 0 is faster than GetSchemaTable and is more future-proofed than SET FMTONLY ON.

Here is my experimental code:

int schemaTime = 0;
int topTime = 0;
int fmtOnTime = 0;

SqlConnection connection = new SqlConnection(@"MyConnectionString");
connection.Open();

SqlCommand schemaCommand = new SqlCommand("SELECT * FROM MyTable", connection);
SqlCommand topCommand = new SqlCommand("SELECT TOP 0 * FROM (SELECT * FROM MyTable) S", connection);
SqlCommand fmtOnCommand = new SqlCommand("SET FMTONLY ON; SELECT * FROM MyTable", connection);

for (int i = 0; i < 1000; i++)
{
    {
        DateTime start = DateTime.Now;
        using (SqlDataReader reader = schemaCommand.ExecuteReader(CommandBehavior.SchemaOnly))
        {
            DataTable table = reader.GetSchemaTable();
        }
        DateTime stop = DateTime.Now;
        TimeSpan span = stop - start;
        schemaTime += span.Milliseconds;
    }

    {
        DateTime start = DateTime.Now;

        DataTable table = new DataTable();
        SqlDataAdapter adapter = new SqlDataAdapter(topCommand);
        adapter.Fill(table);

        DateTime stop = DateTime.Now;
        TimeSpan span = stop - start;
        topTime += span.Milliseconds;
    }

    {
        DateTime start = DateTime.Now;

        DataTable table = new DataTable();
        SqlDataAdapter adapter = new SqlDataAdapter(fmtOnCommand);
        adapter.Fill(table);

        DateTime stop = DateTime.Now;
        TimeSpan span = stop - start;
        fmtOnTime += span.Milliseconds;
    }
}

Console.WriteLine("Schema Time: " + schemaTime);
Console.WriteLine("TOP 0 Time: " + topTime);
Console.WriteLine("FMTONLY ON Time: " + fmtOnTime);

connection.Close();

Answer 1

You could use GetSchemaTable to do what you want.

There is an example of how to use it here.