Question

I want to extracing a column from a database and save it into a list. Now it works but NULL value.
The column vlues in the original database are:

19506
19022
117037
NULL
1680
70659
1666
NULL
NULL
NULL
NULL
NULL
NULL
NULL
NULL
NULL
NULL
8113
NULL

You see there are a lot of “NULL”. I only want to get one “NULL”. My code’s result got many empty strings.

"19506"
"19022"
"117037"
""
"1680"
"70659"
"1666"
""
""
""
""
""
""
""
""
""
""
"8113"
""

What I want is:

"19506"
"19022"
"117037"
""
"1680"
"70659"
"1666"
"8113"

My code:

public static List<string> ExecuteReader(string commandtext,string col)
    {
        List<string> sRetVal = new List<string>();
        try
        {
            using (SqlConnection cnn = new SqlConnection(conn))
            {
                cnn.Open();
                using (SqlCommand cmd = new SqlCommand())
                {
                    cmd.Connection = cnn;
                    cmd.CommandType = CommandType.Text;
                    cmd.CommandText = commandtext;

                    using (SqlDataReader reader = cmd.ExecuteReader())
                    {
                        while (reader.Read())
                        {
                            sRetVal.Add(reader[col].ToString());
                        }

                    }
                }
                cnn.Close();
            }
        }
        catch (Exception ex)
        {

        }
        return sRetVal.ToList().Distinct().ToList();
    }

Thanks.
UPDATED:
string commandtext = "select QO FROM [T1]";

Answer 1

Do the DISTINCT in the database, and it will handle this for you automatically:

select DISTINCT COALESCE(QO,'NULL') FROM [T1]

For the DISTINCT keyword, null values are considered to be duplicates of each other. When DISTINCT is included in a SELECT statement, only one NULL is returned in the results, regardless of how many null values are encountered.

COALESCE:

Returns the first nonnull expression among its arguments.